diff mbox series

[v18,2/2] tools/libxl: VM forking toolstack side

Message ID b91c338ab8165b6e228b46bbd1853eb140ab69c7.1588772376.git.tamas.lengyel@intel.com (mailing list archive)
State New, archived
Headers show
Series [v18,1/2] tools/libxc: add VM forking functions | expand

Commit Message

Tamas K Lengyel May 6, 2020, 1:41 p.m. UTC
Add necessary bits to implement "xl fork-vm" commands. The command allows the
user to specify how to launch the device model allowing for a late-launch model
in which the user can execute the fork without the device model and decide to
only later launch it.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@intel.com>
---
 docs/man/xl.1.pod.in         |  49 +++++
 tools/libxl/libxl.h          |  12 ++
 tools/libxl/libxl_create.c   | 359 ++++++++++++++++++++---------------
 tools/libxl/libxl_dm.c       |   2 +-
 tools/libxl/libxl_dom.c      |  43 ++++-
 tools/libxl/libxl_internal.h |   7 +
 tools/libxl/libxl_types.idl  |   1 +
 tools/libxl/libxl_x86.c      |  42 ++++
 tools/xl/Makefile            |   2 +-
 tools/xl/xl.h                |   5 +
 tools/xl/xl_cmdtable.c       |  15 ++
 tools/xl/xl_forkvm.c         | 149 +++++++++++++++
 tools/xl/xl_vmcontrol.c      |  14 ++
 13 files changed, 535 insertions(+), 165 deletions(-)
 create mode 100644 tools/xl/xl_forkvm.c

Comments

Ian Jackson May 14, 2020, 2:50 p.m. UTC | #1
Tamas K Lengyel writes ("[PATCH v18 2/2] tools/libxl: VM forking toolstack side"):
> Add necessary bits to implement "xl fork-vm" commands. The command allows the
> user to specify how to launch the device model allowing for a late-launch model
> in which the user can execute the fork without the device model and decide to
> only later launch it.

Hi.

Sorry to be so late in reviewing this.  I will divert my main
attention to the API elements...

> +=item B<fork-vm> [I<OPTIONS>] I<domain-id>
> +
> +Create a fork of a running VM.  The domain will be paused after the operation
> +and remains paused while forks of it exist.

Do you mean "must remain paused" ?  And "The original domain" rather
than "The domain" ?

> +B<OPTIONS>
> +
> +=over 4
> +
> +=item B<-p>
> +
> +Leave the fork paused after creating it.

By default the fork runs right away, then, I take it.

> +=item B<--launch-dm>
> +
> +Specify whether the device model (QEMU) should be launched for the fork. Late
> +launch allows to start the device model for an already running fork.

It's not clear to me whether this launches the DM for an existing
fork, or specify when forking that the DM should be run ?

Do you really mean that you can run a fork for a while with no DM ?
How does that work ?

Also you seem to have not documented the launch-dm operation ?

> +=item B<-C>
> +
> +The config file to use when launching the device model.  Currently required when
> +launching the device model.  Most config settings MUST match the parent domain
> +exactly, only change VM name, disk path and network configurations.

This is a libxl config file, right ?

> +=item B<-Q>
> +
> +The path to the qemu save file to use when launching the device model.  Currently
> +required when launching the device model.

Where would the user get one of these ?

I think this question has no good answer and this reveals a problem
with the API...

> +=item B<--fork-reset>
> +
> +Perform a reset operation of an already running fork.  Note that resetting may
> +be less performant then creating a new fork depending on how much memory the
> +fork has deduplicated during its runtime.

What is the semantic effect of a reset ?

> +=item B<--max-vcpus>
> +
> +Specify the max-vcpus matching the parent domain when not launching the dm.

What ?  This makes little sense to me.  You specify vm-fork
--max-vcpus and it changes the parent's max-vcpus ??

> +=item B<--allow-iommu>
> +
> +Specify to allow forking a domain that has IOMMU enabled. Only compatible with
> +forks using --launch-dm no.

Are there no some complex implications here ?  Maybe this doc needs a
caveat.

> +int libxl_domain_fork_launch_dm(libxl_ctx *ctx, libxl_domain_config *d_config,
> +                                uint32_t domid,
> +                                const libxl_asyncprogress_how *aop_console_how)
> +                                LIBXL_EXTERNAL_CALLERS_ONLY;
> +
> +int libxl_domain_fork_reset(libxl_ctx *ctx, uint32_t domid)
> +                            LIBXL_EXTERNAL_CALLERS_ONLY;
>  #endif

I'm afraid I found the code very hard to review.  In particular:

> -    if (!soft_reset) {
> -        struct xen_domctl_createdomain create = {
> -            .ssidref = info->ssidref,
> -            .max_vcpus = b_info->max_vcpus,
> -            .max_evtchn_port = b_info->event_channels,
> -            .max_grant_frames = b_info->max_grant_frames,
> -            .max_maptrack_frames = b_info->max_maptrack_frames,

I think this containsw a lot of code motion.  There is probably some
other refactoring.


Can you please split this up into several patches ?  Code motion
should cocur in patches that do nothing else.  Refactoring should be
broken down into pieces as small as possible, and separated from the
addition of new functionality.  So most of the patches should be
annotated "no functional change".

Thanks,
Ian.
Tamas K Lengyel May 14, 2020, 3:12 p.m. UTC | #2
On Thu, May 14, 2020 at 8:52 AM Ian Jackson <ian.jackson@citrix.com> wrote:
>
> Tamas K Lengyel writes ("[PATCH v18 2/2] tools/libxl: VM forking toolstack side"):
> > Add necessary bits to implement "xl fork-vm" commands. The command allows the
> > user to specify how to launch the device model allowing for a late-launch model
> > in which the user can execute the fork without the device model and decide to
> > only later launch it.
>
> Hi.
>
> Sorry to be so late in reviewing this.  I will divert my main
> attention to the API elements...
>
> > +=item B<fork-vm> [I<OPTIONS>] I<domain-id>
> > +
> > +Create a fork of a running VM.  The domain will be paused after the operation
> > +and remains paused while forks of it exist.
>
> Do you mean "must remain paused" ?  And "The original domain" rather
> than "The domain" ?

Yes, I mean the original domain.

>
> > +B<OPTIONS>
> > +
> > +=over 4
> > +
> > +=item B<-p>
> > +
> > +Leave the fork paused after creating it.
>
> By default the fork runs right away, then, I take it.

Same route is taken as when you run "xl restore" so yes. This applies
if you are launching the device model. Without the device model launch
we are not going down the same path as "xl restore" so the fork is
paused in that case.

>
> > +=item B<--launch-dm>
> > +
> > +Specify whether the device model (QEMU) should be launched for the fork. Late
> > +launch allows to start the device model for an already running fork.
>
> It's not clear to me whether this launches the DM for an existing
> fork, or specify when forking that the DM should be run ?

It's possible to do both. You can create a fork and launch the device
model for it right away, or you can create a fork, unpause it, and
only launch the device model when its actually necessary.

>
> Do you really mean that you can run a fork for a while with no DM ?
> How does that work ?
>
> Also you seem to have not documented the launch-dm operation ?

It's possible I missed it.

>
> > +=item B<-C>
> > +
> > +The config file to use when launching the device model.  Currently required when
> > +launching the device model.  Most config settings MUST match the parent domain
> > +exactly, only change VM name, disk path and network configurations.
>
> This is a libxl config file, right ?

Yes.

>
> > +=item B<-Q>
> > +
> > +The path to the qemu save file to use when launching the device model.  Currently
> > +required when launching the device model.
>
> Where would the user get one of these ?

Generate it by connecting to the qmp socket of the parent domain and
issuing the command saves it. See the cover letter to the series. I
stopped sending the cover letter since there is only this one
outstanding patch now.

>
> I think this question has no good answer and this reveals a problem
> with the API...

I don't know what "problem" you are referring to. We deliberately
chose not to include saving the qemu save file every time a fork is
made because for our usecase you only need to generate the qemu save
file once. Doing it for every fork is huge waste of time since we are
spinning off forks from the same state hundreds of thousands of time.
No need to regenerate the same save file for each.

>
> > +=item B<--fork-reset>
> > +
> > +Perform a reset operation of an already running fork.  Note that resetting may
> > +be less performant then creating a new fork depending on how much memory the
> > +fork has deduplicated during its runtime.
>
> What is the semantic effect of a reset ?

I don't understand the question.

>
> > +=item B<--max-vcpus>
> > +
> > +Specify the max-vcpus matching the parent domain when not launching the dm.
>
> What ?  This makes little sense to me.  You specify vm-fork
> --max-vcpus and it changes the parent's max-vcpus ??

No. You need the max-vcpus value when you create a fork. The domain
create hypercall needs it to set the domain up. I originally wanted to
extend the domain create hypercall so this could be copied by the
hypervisor but the hypervisor maintainers were against changing that
hypercall. So we are left with having to pass it manually.

>
> > +=item B<--allow-iommu>
> > +
> > +Specify to allow forking a domain that has IOMMU enabled. Only compatible with
> > +forks using --launch-dm no.
>
> Are there no some complex implications here ?  Maybe this doc needs a
> caveat.

Only caveat is that this option is only available for forks that have
no device models launched for them. We use it for fuzzing the device
driver of the IOMMU device in the forks.

>
> > +int libxl_domain_fork_launch_dm(libxl_ctx *ctx, libxl_domain_config *d_config,
> > +                                uint32_t domid,
> > +                                const libxl_asyncprogress_how *aop_console_how)
> > +                                LIBXL_EXTERNAL_CALLERS_ONLY;
> > +
> > +int libxl_domain_fork_reset(libxl_ctx *ctx, uint32_t domid)
> > +                            LIBXL_EXTERNAL_CALLERS_ONLY;
> >  #endif
>
> I'm afraid I found the code very hard to review.  In particular:
>
> > -    if (!soft_reset) {
> > -        struct xen_domctl_createdomain create = {
> > -            .ssidref = info->ssidref,
> > -            .max_vcpus = b_info->max_vcpus,
> > -            .max_evtchn_port = b_info->event_channels,
> > -            .max_grant_frames = b_info->max_grant_frames,
> > -            .max_maptrack_frames = b_info->max_maptrack_frames,
>
> I think this containsw a lot of code motion.  There is probably some
> other refactoring.
>
>
> Can you please split this up into several patches ?  Code motion
> should cocur in patches that do nothing else.  Refactoring should be
> broken down into pieces as small as possible, and separated from the
> addition of new functionality.  So most of the patches should be
> annotated "no functional change".

I understand that this patch mixes code-movement and new functionality
which makes it harder for review. The code movement involves no
functional changes to existing code, only splitting some existing
functions to have parts of them reusable for the fork case and skip
the parts that are not relevant. Unfortunately I won't be able to
split this up into multiple patches as I found this codebase
impossibly hard to work with to begin with. It's callback hell with
like 15 different structures being created and freed left and right.
Add to it that it constantly changes even just in the last couple
months, so just rebasing it has been a massive pain. Forget about
backporting this patch too, everything else I was able to easily apply
to Xen 4.13, libxl is just an absolute no go since it has so much code
churn. So since this feature is not required for any of our use-cases
- this is only to make it possible for other potential users to use
forks where they need them to be more fully featured - I won't be able
to assign the time it would require to split this up further. It
probably would take me weeks which I can't justify at the moment.

Tamas
diff mbox series

Patch

diff --git a/docs/man/xl.1.pod.in b/docs/man/xl.1.pod.in
index 09339282e6..67b4e8588a 100644
--- a/docs/man/xl.1.pod.in
+++ b/docs/man/xl.1.pod.in
@@ -708,6 +708,55 @@  above).
 
 =back
 
+=item B<fork-vm> [I<OPTIONS>] I<domain-id>
+
+Create a fork of a running VM.  The domain will be paused after the operation
+and remains paused while forks of it exist.  Experimental and x86 only.
+Forks can only be made of domains with HAP enabled and on Intel hardware.  The
+parent domain must be created with the xl toolstack and its configuration must
+not manually define max_grant_frames, max_maptrack_frames or max_event_channels.
+
+B<OPTIONS>
+
+=over 4
+
+=item B<-p>
+
+Leave the fork paused after creating it.
+
+=item B<--launch-dm>
+
+Specify whether the device model (QEMU) should be launched for the fork. Late
+launch allows to start the device model for an already running fork.
+
+=item B<-C>
+
+The config file to use when launching the device model.  Currently required when
+launching the device model.  Most config settings MUST match the parent domain
+exactly, only change VM name, disk path and network configurations.
+
+=item B<-Q>
+
+The path to the qemu save file to use when launching the device model.  Currently
+required when launching the device model.
+
+=item B<--fork-reset>
+
+Perform a reset operation of an already running fork.  Note that resetting may
+be less performant then creating a new fork depending on how much memory the
+fork has deduplicated during its runtime.
+
+=item B<--max-vcpus>
+
+Specify the max-vcpus matching the parent domain when not launching the dm.
+
+=item B<--allow-iommu>
+
+Specify to allow forking a domain that has IOMMU enabled. Only compatible with
+forks using --launch-dm no.
+
+=back
+
 =item B<sharing> [I<domain-id>]
 
 Display the number of shared pages for a specified domain. If no domain is
diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
index 71709dc585..4bbb0a773d 100644
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -2666,6 +2666,18 @@  int libxl_psr_get_hw_info(libxl_ctx *ctx, libxl_psr_feat_type type,
                           unsigned int lvl, unsigned int *nr,
                           libxl_psr_hw_info **info);
 void libxl_psr_hw_info_list_free(libxl_psr_hw_info *list, unsigned int nr);
+
+int libxl_domain_fork_vm(libxl_ctx *ctx, uint32_t pdomid, uint32_t max_vcpus,
+                         bool allow_with_iommu, uint32_t *domid)
+                         LIBXL_EXTERNAL_CALLERS_ONLY;
+
+int libxl_domain_fork_launch_dm(libxl_ctx *ctx, libxl_domain_config *d_config,
+                                uint32_t domid,
+                                const libxl_asyncprogress_how *aop_console_how)
+                                LIBXL_EXTERNAL_CALLERS_ONLY;
+
+int libxl_domain_fork_reset(libxl_ctx *ctx, uint32_t domid)
+                            LIBXL_EXTERNAL_CALLERS_ONLY;
 #endif
 
 /* misc */
diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index 5a043df15f..1a930c2de7 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -538,12 +538,12 @@  out:
     return ret;
 }
 
-int libxl__domain_make(libxl__gc *gc, libxl_domain_config *d_config,
-                       libxl__domain_build_state *state,
-                       uint32_t *domid, bool soft_reset)
+static int libxl__domain_make_xs_entries(libxl__gc *gc, libxl_domain_config *d_config,
+                                         libxl__domain_build_state *state,
+                                         uint32_t domid)
 {
     libxl_ctx *ctx = libxl__gc_owner(gc);
-    int ret, rc, nb_vm;
+    int rc, nb_vm;
     const char *dom_type;
     char *uuid_string;
     char *dom_path, *vm_path, *libxl_path;
@@ -555,9 +555,6 @@  int libxl__domain_make(libxl__gc *gc, libxl_domain_config *d_config,
 
     /* convenience aliases */
     libxl_domain_create_info *info = &d_config->c_info;
-    libxl_domain_build_info *b_info = &d_config->b_info;
-
-    assert(soft_reset || *domid == INVALID_DOMID);
 
     uuid_string = libxl__uuid2string(gc, info->uuid);
     if (!uuid_string) {
@@ -565,137 +562,7 @@  int libxl__domain_make(libxl__gc *gc, libxl_domain_config *d_config,
         goto out;
     }
 
-    if (!soft_reset) {
-        struct xen_domctl_createdomain create = {
-            .ssidref = info->ssidref,
-            .max_vcpus = b_info->max_vcpus,
-            .max_evtchn_port = b_info->event_channels,
-            .max_grant_frames = b_info->max_grant_frames,
-            .max_maptrack_frames = b_info->max_maptrack_frames,
-        };
-
-        if (info->type != LIBXL_DOMAIN_TYPE_PV) {
-            create.flags |= XEN_DOMCTL_CDF_hvm;
-            create.flags |=
-                libxl_defbool_val(info->hap) ? XEN_DOMCTL_CDF_hap : 0;
-            create.flags |=
-                libxl_defbool_val(info->oos) ? 0 : XEN_DOMCTL_CDF_oos_off;
-        }
-
-        assert(info->passthrough != LIBXL_PASSTHROUGH_DEFAULT);
-        LOG(DETAIL, "passthrough: %s",
-            libxl_passthrough_to_string(info->passthrough));
-
-        if (info->passthrough != LIBXL_PASSTHROUGH_DISABLED)
-            create.flags |= XEN_DOMCTL_CDF_iommu;
-
-        if (info->passthrough == LIBXL_PASSTHROUGH_SYNC_PT)
-            create.iommu_opts |= XEN_DOMCTL_IOMMU_no_sharept;
-
-        /* Ultimately, handle is an array of 16 uint8_t, same as uuid */
-        libxl_uuid_copy(ctx, (libxl_uuid *)&create.handle, &info->uuid);
-
-        ret = libxl__arch_domain_prepare_config(gc, d_config, &create);
-        if (ret < 0) {
-            LOGED(ERROR, *domid, "fail to get domain config");
-            rc = ERROR_FAIL;
-            goto out;
-        }
-
-        for (;;) {
-            uint32_t local_domid;
-            bool recent;
-
-            if (info->domid == RANDOM_DOMID) {
-                uint16_t v;
-
-                ret = libxl__random_bytes(gc, (void *)&v, sizeof(v));
-                if (ret < 0)
-                    break;
-
-                v &= DOMID_MASK;
-                if (!libxl_domid_valid_guest(v))
-                    continue;
-
-                local_domid = v;
-            } else {
-                local_domid = info->domid; /* May not be valid */
-            }
-
-            ret = xc_domain_create(ctx->xch, &local_domid, &create);
-            if (ret < 0) {
-                /*
-                 * If we generated a random domid and creation failed
-                 * because that domid already exists then simply try
-                 * again.
-                 */
-                if (errno == EEXIST && info->domid == RANDOM_DOMID)
-                    continue;
-
-                LOGED(ERROR, local_domid, "domain creation fail");
-                rc = ERROR_FAIL;
-                goto out;
-            }
-
-            /* A new domain now exists */
-            *domid = local_domid;
-
-            rc = libxl__is_domid_recent(gc, local_domid, &recent);
-            if (rc)
-                goto out;
-
-            /* The domid is not recent, so we're done */
-            if (!recent)
-                break;
-
-            /*
-             * If the domid was specified then there's no point in
-             * trying again.
-             */
-            if (libxl_domid_valid_guest(info->domid)) {
-                LOGED(ERROR, local_domid, "domain id recently used");
-                rc = ERROR_FAIL;
-                goto out;
-            }
-
-            /*
-             * The domain is recent and so cannot be used. Clear domid
-             * here since, if xc_domain_destroy() fails below there is
-             * little point calling it again in the error path.
-             */
-            *domid = INVALID_DOMID;
-
-            ret = xc_domain_destroy(ctx->xch, local_domid);
-            if (ret < 0) {
-                LOGED(ERROR, local_domid, "domain destroy fail");
-                rc = ERROR_FAIL;
-                goto out;
-            }
-
-            /* The domain was successfully destroyed, so we can try again */
-        }
-
-        rc = libxl__arch_domain_save_config(gc, d_config, state, &create);
-        if (rc < 0)
-            goto out;
-    }
-
-    /*
-     * If soft_reset is set the the domid will have been valid on entry.
-     * If it was not set then xc_domain_create() should have assigned a
-     * valid value. Either way, if we reach this point, domid should be
-     * valid.
-     */
-    assert(libxl_domid_valid_guest(*domid));
-
-    ret = xc_cpupool_movedomain(ctx->xch, info->poolid, *domid);
-    if (ret < 0) {
-        LOGED(ERROR, *domid, "domain move fail");
-        rc = ERROR_FAIL;
-        goto out;
-    }
-
-    dom_path = libxl__xs_get_dompath(gc, *domid);
+    dom_path = libxl__xs_get_dompath(gc, domid);
     if (!dom_path) {
         rc = ERROR_FAIL;
         goto out;
@@ -703,12 +570,12 @@  int libxl__domain_make(libxl__gc *gc, libxl_domain_config *d_config,
 
     vm_path = GCSPRINTF("/vm/%s", uuid_string);
     if (!vm_path) {
-        LOGD(ERROR, *domid, "cannot allocate create paths");
+        LOGD(ERROR, domid, "cannot allocate create paths");
         rc = ERROR_FAIL;
         goto out;
     }
 
-    libxl_path = libxl__xs_libxl_path(gc, *domid);
+    libxl_path = libxl__xs_libxl_path(gc, domid);
     if (!libxl_path) {
         rc = ERROR_FAIL;
         goto out;
@@ -719,10 +586,10 @@  int libxl__domain_make(libxl__gc *gc, libxl_domain_config *d_config,
 
     roperm[0].id = 0;
     roperm[0].perms = XS_PERM_NONE;
-    roperm[1].id = *domid;
+    roperm[1].id = domid;
     roperm[1].perms = XS_PERM_READ;
 
-    rwperm[0].id = *domid;
+    rwperm[0].id = domid;
     rwperm[0].perms = XS_PERM_NONE;
 
 retry_transaction:
@@ -740,7 +607,7 @@  retry_transaction:
                     noperm, ARRAY_SIZE(noperm));
 
     xs_write(ctx->xsh, t, GCSPRINTF("%s/vm", dom_path), vm_path, strlen(vm_path));
-    rc = libxl__domain_rename(gc, *domid, 0, info->name, t);
+    rc = libxl__domain_rename(gc, domid, 0, info->name, t);
     if (rc)
         goto out;
 
@@ -830,7 +697,7 @@  retry_transaction:
 
     vm_list = libxl_list_vm(ctx, &nb_vm);
     if (!vm_list) {
-        LOGD(ERROR, *domid, "cannot get number of running guests");
+        LOGD(ERROR, domid, "cannot get number of running guests");
         rc = ERROR_FAIL;
         goto out;
     }
@@ -854,7 +721,7 @@  retry_transaction:
             t = 0;
             goto retry_transaction;
         }
-        LOGED(ERROR, *domid, "domain creation ""xenstore transaction commit failed");
+        LOGED(ERROR, domid, "domain creation ""xenstore transaction commit failed");
         rc = ERROR_FAIL;
         goto out;
     }
@@ -866,6 +733,155 @@  retry_transaction:
     return rc;
 }
 
+int libxl__domain_make(libxl__gc *gc, libxl_domain_config *d_config,
+                       libxl__domain_build_state *state,
+                       uint32_t *domid, bool soft_reset)
+{
+    libxl_ctx *ctx = libxl__gc_owner(gc);
+    int ret, rc;
+
+    /* convenience aliases */
+    libxl_domain_create_info *info = &d_config->c_info;
+    libxl_domain_build_info *b_info = &d_config->b_info;
+
+    assert(soft_reset || *domid == INVALID_DOMID);
+
+    if (!soft_reset) {
+        struct xen_domctl_createdomain create = {
+            .ssidref = info->ssidref,
+            .max_vcpus = b_info->max_vcpus,
+            .max_evtchn_port = b_info->event_channels,
+            .max_grant_frames = b_info->max_grant_frames,
+            .max_maptrack_frames = b_info->max_maptrack_frames,
+        };
+
+        if (info->type != LIBXL_DOMAIN_TYPE_PV) {
+            create.flags |= XEN_DOMCTL_CDF_hvm;
+            create.flags |=
+                libxl_defbool_val(info->hap) ? XEN_DOMCTL_CDF_hap : 0;
+            create.flags |=
+                libxl_defbool_val(info->oos) ? 0 : XEN_DOMCTL_CDF_oos_off;
+        }
+
+        assert(info->passthrough != LIBXL_PASSTHROUGH_DEFAULT);
+        LOG(DETAIL, "passthrough: %s",
+            libxl_passthrough_to_string(info->passthrough));
+
+        if (info->passthrough != LIBXL_PASSTHROUGH_DISABLED)
+            create.flags |= XEN_DOMCTL_CDF_iommu;
+
+        if (info->passthrough == LIBXL_PASSTHROUGH_SYNC_PT)
+            create.iommu_opts |= XEN_DOMCTL_IOMMU_no_sharept;
+
+        /* Ultimately, handle is an array of 16 uint8_t, same as uuid */
+        libxl_uuid_copy(ctx, (libxl_uuid *)&create.handle, &info->uuid);
+
+        ret = libxl__arch_domain_prepare_config(gc, d_config, &create);
+        if (ret < 0) {
+            LOGED(ERROR, *domid, "fail to get domain config");
+            rc = ERROR_FAIL;
+            goto out;
+        }
+
+        for (;;) {
+            uint32_t local_domid;
+            bool recent;
+
+            if (info->domid == RANDOM_DOMID) {
+                uint16_t v;
+
+                ret = libxl__random_bytes(gc, (void *)&v, sizeof(v));
+                if (ret < 0)
+                    break;
+
+                v &= DOMID_MASK;
+                if (!libxl_domid_valid_guest(v))
+                    continue;
+
+                local_domid = v;
+            } else {
+                local_domid = info->domid; /* May not be valid */
+            }
+
+            ret = xc_domain_create(ctx->xch, &local_domid, &create);
+            if (ret < 0) {
+                /*
+                 * If we generated a random domid and creation failed
+                 * because that domid already exists then simply try
+                 * again.
+                 */
+                if (errno == EEXIST && info->domid == RANDOM_DOMID)
+                    continue;
+
+                LOGED(ERROR, local_domid, "domain creation fail");
+                rc = ERROR_FAIL;
+                goto out;
+            }
+
+            /* A new domain now exists */
+            *domid = local_domid;
+
+            rc = libxl__is_domid_recent(gc, local_domid, &recent);
+            if (rc)
+                goto out;
+
+            /* The domid is not recent, so we're done */
+            if (!recent)
+                break;
+
+            /*
+             * If the domid was specified then there's no point in
+             * trying again.
+             */
+            if (libxl_domid_valid_guest(info->domid)) {
+                LOGED(ERROR, local_domid, "domain id recently used");
+                rc = ERROR_FAIL;
+                goto out;
+            }
+
+            /*
+             * The domain is recent and so cannot be used. Clear domid
+             * here since, if xc_domain_destroy() fails below there is
+             * little point calling it again in the error path.
+             */
+            *domid = INVALID_DOMID;
+
+            ret = xc_domain_destroy(ctx->xch, local_domid);
+            if (ret < 0) {
+                LOGED(ERROR, local_domid, "domain destroy fail");
+                rc = ERROR_FAIL;
+                goto out;
+            }
+
+            /* The domain was successfully destroyed, so we can try again */
+        }
+
+        rc = libxl__arch_domain_save_config(gc, d_config, state, &create);
+        if (rc < 0)
+            goto out;
+    }
+
+    /*
+     * If soft_reset is set the the domid will have been valid on entry.
+     * If it was not set then xc_domain_create() should have assigned a
+     * valid value. Either way, if we reach this point, domid should be
+     * valid.
+     */
+    assert(libxl_domid_valid_guest(*domid));
+
+    ret = xc_cpupool_movedomain(ctx->xch, info->poolid, *domid);
+    if (ret < 0) {
+        LOGED(ERROR, *domid, "domain move fail");
+        rc = ERROR_FAIL;
+        goto out;
+    }
+
+    rc = libxl__domain_make_xs_entries(gc, d_config, state, *domid);
+
+out:
+    return rc;
+}
+
 static int store_libxl_entry(libxl__gc *gc, uint32_t domid,
                              libxl_domain_build_info *b_info)
 {
@@ -1192,15 +1208,31 @@  static void initiate_domain_create(libxl__egc *egc,
     ret = libxl__domain_config_setdefault(gc,d_config,domid);
     if (ret) goto error_out;
 
-    ret = libxl__domain_make(gc, d_config, dbs, &domid, dcs->soft_reset);
-    if (ret) {
-        LOGD(ERROR, domid, "cannot make domain: %d", ret);
+    if (!d_config->dm_restore_file)
+    {
+        ret = libxl__domain_make(gc, d_config, dbs, &domid, dcs->soft_reset);
+        if (ret) {
+            LOGD(ERROR, domid, "cannot make domain: %d", ret);
+            ret = ERROR_FAIL;
+            goto error_out;
+        }
+
         dcs->guest_domid = domid;
+    } else if (dcs->guest_domid != INVALID_DOMID) {
+        domid = dcs->guest_domid;
+
+        ret = libxl__domain_make_xs_entries(gc, d_config, &dcs->build_state, domid);
+        if (ret) {
+            LOGD(ERROR, domid, "cannot make domain: %d", ret);
+            ret = ERROR_FAIL;
+            goto error_out;
+        }
+    } else {
+        LOGD(ERROR, domid, "cannot make domain");
         ret = ERROR_FAIL;
         goto error_out;
     }
 
-    dcs->guest_domid = domid;
     dcs->sdss.dm.guest_domid = 0; /* means we haven't spawned */
 
     /* post-4.13 todo: move these next bits of defaulting to
@@ -1236,7 +1268,7 @@  static void initiate_domain_create(libxl__egc *egc,
     if (ret)
         goto error_out;
 
-    if (dbs->restore || dcs->soft_reset) {
+    if (dbs->restore || dcs->soft_reset || d_config->dm_restore_file) {
         LOGD(DEBUG, domid, "restoring, not running bootloader");
         domcreate_bootloader_done(egc, &dcs->bl, 0);
     } else  {
@@ -1312,7 +1344,16 @@  static void domcreate_bootloader_done(libxl__egc *egc,
     dcs->sdss.dm.callback = domcreate_devmodel_started;
     dcs->sdss.callback = domcreate_devmodel_started;
 
-    if (restore_fd < 0 && !dcs->soft_reset) {
+    if (restore_fd < 0 && !dcs->soft_reset && !d_config->dm_restore_file) {
+        rc = libxl__domain_build(gc, d_config, domid, state);
+        domcreate_rebuild_done(egc, dcs, rc);
+        return;
+    }
+
+    if (d_config->dm_restore_file) {
+        dcs->srs.dcs = dcs;
+        dcs->srs.ao = ao;
+        state->forked_vm = true;
         rc = libxl__domain_build(gc, d_config, domid, state);
         domcreate_rebuild_done(egc, dcs, rc);
         return;
@@ -1510,6 +1551,7 @@  static void domcreate_rebuild_done(libxl__egc *egc,
     /* convenience aliases */
     const uint32_t domid = dcs->guest_domid;
     libxl_domain_config *const d_config = dcs->guest_config;
+    libxl__domain_build_state *const state = &dcs->build_state;
 
     if (ret) {
         LOGD(ERROR, domid, "cannot (re-)build domain: %d", ret);
@@ -1517,6 +1559,9 @@  static void domcreate_rebuild_done(libxl__egc *egc,
         goto error_out;
     }
 
+    if (d_config->dm_restore_file)
+        state->saved_state = GCSPRINTF("%s", d_config->dm_restore_file);
+
     store_libxl_entry(gc, domid, &d_config->b_info);
 
     libxl__multidev_begin(ao, &dcs->multidev);
@@ -1947,7 +1992,7 @@  static void domain_create_cb(libxl__egc *egc,
                              libxl__domain_create_state *dcs,
                              int rc, uint32_t domid);
 
-static int do_domain_create(libxl_ctx *ctx, libxl_domain_config *d_config,
+int libxl__do_domain_create(libxl_ctx *ctx, libxl_domain_config *d_config,
                             uint32_t *domid, int restore_fd, int send_back_fd,
                             const libxl_domain_restore_params *params,
                             const libxl_asyncop_how *ao_how,
@@ -1960,6 +2005,8 @@  static int do_domain_create(libxl_ctx *ctx, libxl_domain_config *d_config,
     GCNEW(cdcs);
     cdcs->dcs.ao = ao;
     cdcs->dcs.guest_config = d_config;
+    cdcs->dcs.guest_domid = *domid;
+
     libxl_domain_config_init(&cdcs->dcs.guest_config_saved);
     libxl_domain_config_copy(ctx, &cdcs->dcs.guest_config_saved, d_config);
     cdcs->dcs.restore_fd = cdcs->dcs.libxc_fd = restore_fd;
@@ -2204,8 +2251,8 @@  int libxl_domain_create_new(libxl_ctx *ctx, libxl_domain_config *d_config,
                             const libxl_asyncprogress_how *aop_console_how)
 {
     unset_disk_colo_restore(d_config);
-    return do_domain_create(ctx, d_config, domid, -1, -1, NULL,
-                            ao_how, aop_console_how);
+    return libxl__do_domain_create(ctx, d_config, domid, -1, -1, NULL,
+                                  ao_how, aop_console_how);
 }
 
 int libxl_domain_create_restore(libxl_ctx *ctx, libxl_domain_config *d_config,
@@ -2221,8 +2268,8 @@  int libxl_domain_create_restore(libxl_ctx *ctx, libxl_domain_config *d_config,
         unset_disk_colo_restore(d_config);
     }
 
-    return do_domain_create(ctx, d_config, domid, restore_fd, send_back_fd,
-                            params, ao_how, aop_console_how);
+    return libxl__do_domain_create(ctx, d_config, domid, restore_fd, send_back_fd,
+                                   params, ao_how, aop_console_how);
 }
 
 int libxl_domain_soft_reset(libxl_ctx *ctx,
diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
index f4007bbe50..b615f1fc88 100644
--- a/tools/libxl/libxl_dm.c
+++ b/tools/libxl/libxl_dm.c
@@ -2803,7 +2803,7 @@  static void device_model_spawn_outcome(libxl__egc *egc,
 
     libxl__domain_build_state *state = dmss->build_state;
 
-    if (state->saved_state) {
+    if (state->saved_state && !state->forked_vm) {
         ret2 = unlink(state->saved_state);
         if (ret2) {
             LOGED(ERROR, dmss->guest_domid, "%s: failed to remove device-model state %s",
diff --git a/tools/libxl/libxl_dom.c b/tools/libxl/libxl_dom.c
index 71cb578923..9e47162f67 100644
--- a/tools/libxl/libxl_dom.c
+++ b/tools/libxl/libxl_dom.c
@@ -249,9 +249,12 @@  int libxl__build_pre(libxl__gc *gc, uint32_t domid,
     libxl_domain_build_info *const info = &d_config->b_info;
     libxl_ctx *ctx = libxl__gc_owner(gc);
     char *xs_domid, *con_domid;
-    int rc;
+    int rc = 0;
     uint64_t size;
 
+    if (state->forked_vm)
+        goto skip_fork;
+
     if (xc_domain_max_vcpus(ctx->xch, domid, info->max_vcpus) != 0) {
         LOG(ERROR, "Couldn't set max vcpu count");
         return ERROR_FAIL;
@@ -362,7 +365,6 @@  int libxl__build_pre(libxl__gc *gc, uint32_t domid,
         }
     }
 
-
     rc = libxl__arch_extra_memory(gc, info, &size);
     if (rc < 0) {
         LOGE(ERROR, "Couldn't get arch extra constant memory size");
@@ -374,6 +376,11 @@  int libxl__build_pre(libxl__gc *gc, uint32_t domid,
         return ERROR_FAIL;
     }
 
+    rc = libxl__arch_domain_create(gc, d_config, domid);
+    if (rc)
+        goto out;
+
+skip_fork:
     xs_domid = xs_read(ctx->xsh, XBT_NULL, "/tool/xenstored/domid", NULL);
     state->store_domid = xs_domid ? atoi(xs_domid) : 0;
     free(xs_domid);
@@ -385,8 +392,7 @@  int libxl__build_pre(libxl__gc *gc, uint32_t domid,
     state->store_port = xc_evtchn_alloc_unbound(ctx->xch, domid, state->store_domid);
     state->console_port = xc_evtchn_alloc_unbound(ctx->xch, domid, state->console_domid);
 
-    rc = libxl__arch_domain_create(gc, d_config, domid);
-
+out:
     return rc;
 }
 
@@ -444,6 +450,9 @@  int libxl__build_post(libxl__gc *gc, uint32_t domid,
     char **ents;
     int i, rc;
 
+    if (state->forked_vm)
+        goto skip_fork;
+
     if (info->num_vnuma_nodes && !info->num_vcpu_soft_affinity) {
         rc = set_vnuma_affinity(gc, domid, info);
         if (rc)
@@ -466,6 +475,7 @@  int libxl__build_post(libxl__gc *gc, uint32_t domid,
         }
     }
 
+skip_fork:
     ents = libxl__calloc(gc, 12 + (info->max_vcpus * 2) + 2, sizeof(char *));
     ents[0] = "memory/static-max";
     ents[1] = GCSPRINTF("%"PRId64, info->max_memkb);
@@ -728,14 +738,16 @@  static int hvm_build_set_params(xc_interface *handle, uint32_t domid,
                                 libxl_domain_build_info *info,
                                 int store_evtchn, unsigned long *store_mfn,
                                 int console_evtchn, unsigned long *console_mfn,
-                                domid_t store_domid, domid_t console_domid)
+                                domid_t store_domid, domid_t console_domid,
+                                bool forked_vm)
 {
     struct hvm_info_table *va_hvm;
     uint8_t *va_map, sum;
     uint64_t str_mfn, cons_mfn;
     int i;
 
-    if (info->type == LIBXL_DOMAIN_TYPE_HVM) {
+    if (info->type == LIBXL_DOMAIN_TYPE_HVM && !forked_vm)
+    {
         va_map = xc_map_foreign_range(handle, domid,
                                       XC_PAGE_SIZE, PROT_READ | PROT_WRITE,
                                       HVM_INFO_PFN);
@@ -1051,6 +1063,23 @@  int libxl__build_hvm(libxl__gc *gc, uint32_t domid,
     struct xc_dom_image *dom = NULL;
     bool device_model = info->type == LIBXL_DOMAIN_TYPE_HVM ? true : false;
 
+    if (state->forked_vm)
+    {
+        rc = hvm_build_set_params(ctx->xch, domid, info, state->store_port,
+                                  &state->store_mfn, state->console_port,
+                                  &state->console_mfn, state->store_domid,
+                                  state->console_domid, state->forked_vm);
+
+        if ( rc )
+            return rc;
+
+        return xc_dom_gnttab_seed(ctx->xch, domid, true,
+                                  state->console_mfn,
+                                  state->store_mfn,
+                                  state->console_domid,
+                                  state->store_domid);
+    }
+
     xc_dom_loginit(ctx->xch);
 
     /*
@@ -1175,7 +1204,7 @@  int libxl__build_hvm(libxl__gc *gc, uint32_t domid,
     rc = hvm_build_set_params(ctx->xch, domid, info, state->store_port,
                                &state->store_mfn, state->console_port,
                                &state->console_mfn, state->store_domid,
-                               state->console_domid);
+                               state->console_domid, false);
     if (rc != 0) {
         LOG(ERROR, "hvm build set params failed");
         goto out;
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index e5effd2ad1..b25cb201b1 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -1374,6 +1374,7 @@  typedef struct {
 
     char *saved_state;
     int dm_monitor_fd;
+    bool forked_vm;
 
     libxl__file_reference pv_kernel;
     libxl__file_reference pv_ramdisk;
@@ -4822,6 +4823,12 @@  _hidden int libxl__domain_pvcontrol(libxl__egc *egc,
 /* Check whether a domid is recent */
 int libxl__is_domid_recent(libxl__gc *gc, uint32_t domid, bool *recent);
 
+_hidden int libxl__do_domain_create(libxl_ctx *ctx, libxl_domain_config *d_config,
+                                    uint32_t *domid, int restore_fd, int send_back_fd,
+                                    const libxl_domain_restore_params *params,
+                                    const libxl_asyncop_how *ao_how,
+                                    const libxl_asyncprogress_how *aop_console_how);
+
 #endif
 
 /*
diff --git a/tools/libxl/libxl_types.idl b/tools/libxl/libxl_types.idl
index f7c473be74..2bb5e6319e 100644
--- a/tools/libxl/libxl_types.idl
+++ b/tools/libxl/libxl_types.idl
@@ -958,6 +958,7 @@  libxl_domain_config = Struct("domain_config", [
     ("on_watchdog", libxl_action_on_shutdown),
     ("on_crash", libxl_action_on_shutdown),
     ("on_soft_reset", libxl_action_on_shutdown),
+    ("dm_restore_file", string, {'const': True}),
     ], dir=DIR_IN)
 
 libxl_diskinfo = Struct("diskinfo", [
diff --git a/tools/libxl/libxl_x86.c b/tools/libxl/libxl_x86.c
index f8bc828e62..f6d7daa8fe 100644
--- a/tools/libxl/libxl_x86.c
+++ b/tools/libxl/libxl_x86.c
@@ -2,6 +2,7 @@ 
 #include "libxl_arch.h"
 
 #include <xc_dom.h>
+#include <xen-xsm/flask/flask.h>
 
 int libxl__arch_domain_prepare_config(libxl__gc *gc,
                                       libxl_domain_config *d_config,
@@ -842,6 +843,47 @@  int libxl__arch_passthrough_mode_setdefault(libxl__gc *gc,
     return rc;
 }
 
+/*
+ * The parent domain is expected to be created with default settings for
+ * - max_evtch_port
+ * - max_grant_frames
+ * - max_maptrack_frames
+ */
+int libxl_domain_fork_vm(libxl_ctx *ctx, uint32_t pdomid, uint32_t max_vcpus,
+                         bool allow_with_iommu, uint32_t *domid)
+{
+    int rc;
+    struct xen_domctl_createdomain create = {0};
+    create.flags |= XEN_DOMCTL_CDF_hvm;
+    create.flags |= XEN_DOMCTL_CDF_hap;
+    create.flags |= XEN_DOMCTL_CDF_oos_off;
+    create.arch.emulation_flags = (XEN_X86_EMU_ALL & ~XEN_X86_EMU_VPCI);
+    create.ssidref = SECINITSID_DOMU;
+    create.max_vcpus = max_vcpus;
+    create.max_evtchn_port = 1023;
+    create.max_grant_frames = LIBXL_MAX_GRANT_FRAMES_DEFAULT;
+    create.max_maptrack_frames = LIBXL_MAX_MAPTRACK_FRAMES_DEFAULT;
+
+    if ( (rc = xc_domain_create(ctx->xch, domid, &create)) )
+        return rc;
+
+    if ( (rc = xc_memshr_fork(ctx->xch, pdomid, *domid, allow_with_iommu)) )
+        xc_domain_destroy(ctx->xch, *domid);
+
+    return rc;
+}
+
+int libxl_domain_fork_launch_dm(libxl_ctx *ctx, libxl_domain_config *d_config,
+                                uint32_t domid,
+                                const libxl_asyncprogress_how *aop_console_how)
+{
+    return libxl__do_domain_create(ctx, d_config, &domid, -1, -1, 0, 0, aop_console_how);
+}
+
+int libxl_domain_fork_reset(libxl_ctx *ctx, uint32_t domid)
+{
+    return xc_memshr_fork_reset(ctx->xch, domid);
+}
 
 /*
  * Local variables:
diff --git a/tools/xl/Makefile b/tools/xl/Makefile
index af4912e67a..073222233b 100644
--- a/tools/xl/Makefile
+++ b/tools/xl/Makefile
@@ -15,7 +15,7 @@  LDFLAGS += $(PTHREAD_LDFLAGS)
 CFLAGS_XL += $(CFLAGS_libxenlight)
 CFLAGS_XL += -Wshadow
 
-XL_OBJS-$(CONFIG_X86) = xl_psr.o
+XL_OBJS-$(CONFIG_X86) = xl_psr.o xl_forkvm.o
 XL_OBJS = xl.o xl_cmdtable.o xl_sxp.o xl_utils.o $(XL_OBJS-y)
 XL_OBJS += xl_parse.o xl_cpupool.o xl_flask.o
 XL_OBJS += xl_vtpm.o xl_block.o xl_nic.o xl_usb.o
diff --git a/tools/xl/xl.h b/tools/xl/xl.h
index 06569c6c4a..1105c34b15 100644
--- a/tools/xl/xl.h
+++ b/tools/xl/xl.h
@@ -31,6 +31,7 @@  struct cmd_spec {
 };
 
 struct domain_create {
+    uint32_t ddomid; /* fork launch dm for this domid */
     int debug;
     int daemonize;
     int monitor; /* handle guest reboots etc */
@@ -45,6 +46,7 @@  struct domain_create {
     const char *config_file;
     char *extra_config; /* extra config string */
     const char *restore_file;
+    const char *dm_restore_file;
     char *colo_proxy_script;
     bool userspace_colo_proxy;
     int migrate_fd; /* -1 means none */
@@ -128,6 +130,8 @@  int main_pciassignable_remove(int argc, char **argv);
 int main_pciassignable_list(int argc, char **argv);
 #ifndef LIBXL_HAVE_NO_SUSPEND_RESUME
 int main_restore(int argc, char **argv);
+int main_fork_launch_dm(int argc, char **argv);
+int main_fork_reset(int argc, char **argv);
 int main_migrate_receive(int argc, char **argv);
 int main_save(int argc, char **argv);
 int main_migrate(int argc, char **argv);
@@ -212,6 +216,7 @@  int main_psr_cat_cbm_set(int argc, char **argv);
 int main_psr_cat_show(int argc, char **argv);
 int main_psr_mba_set(int argc, char **argv);
 int main_psr_mba_show(int argc, char **argv);
+int main_fork_vm(int argc, char **argv);
 #endif
 int main_qemu_monitor_command(int argc, char **argv);
 
diff --git a/tools/xl/xl_cmdtable.c b/tools/xl/xl_cmdtable.c
index 08335394e5..ef634abf32 100644
--- a/tools/xl/xl_cmdtable.c
+++ b/tools/xl/xl_cmdtable.c
@@ -187,6 +187,21 @@  struct cmd_spec cmd_table[] = {
       "Restore a domain from a saved state",
       "- for internal use only",
     },
+#if defined(__i386__) || defined(__x86_64__)
+    { "fork-vm",
+      &main_fork_vm, 0, 1,
+      "Fork a domain from the running parent domid. Experimental. Most config settings must match parent.",
+      "[options] <Domid>",
+      "-h                           Print this help.\n"
+      "-C <config>                  Use config file for VM fork.\n"
+      "-Q <qemu-save-file>          Use qemu save file for VM fork.\n"
+      "--launch-dm <yes|no|late>    Launch device model (QEMU) for VM fork.\n"
+      "--fork-reset                 Reset VM fork.\n"
+      "--max-vcpus                  Specify max-vcpus matching the parent domain when not launching dm\n"
+      "-p                           Do not unpause fork VM after operation.\n"
+      "-d                           Enable debug messages.\n"
+    },
+#endif
 #endif
     { "dump-core",
       &main_dump_core, 0, 1,
diff --git a/tools/xl/xl_forkvm.c b/tools/xl/xl_forkvm.c
new file mode 100644
index 0000000000..46805b84f3
--- /dev/null
+++ b/tools/xl/xl_forkvm.c
@@ -0,0 +1,149 @@ 
+/*
+ * Copyright 2020 Intel Corporation
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation; version 2.1 only. with the special
+ * exception on linking described in file LICENSE.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ */
+
+#include <fcntl.h>
+#include <inttypes.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/utsname.h>
+#include <time.h>
+#include <unistd.h>
+
+#include <libxl.h>
+#include <libxl_utils.h>
+#include <libxlutil.h>
+
+#include "xl.h"
+#include "xl_utils.h"
+#include "xl_parse.h"
+
+int main_fork_vm(int argc, char **argv)
+{
+    int rc, debug = 0;
+    uint32_t domid_in = INVALID_DOMID, domid_out = INVALID_DOMID;
+    int launch_dm = 1;
+    bool reset = 0;
+    bool pause = 0;
+    bool allow_iommu = 0;
+    const char *config_file = NULL;
+    const char *dm_restore_file = NULL;
+    uint32_t max_vcpus = 0;
+
+    int opt;
+    static struct option opts[] = {
+        {"launch-dm", 1, 0, 'l'},
+        {"fork-reset", 0, 0, 'r'},
+        {"max-vcpus", 1, 0, 'm'},
+        {"allow-iommu", 1, 0, 'i'},
+        COMMON_LONG_OPTS
+    };
+
+    SWITCH_FOREACH_OPT(opt, "phdC:Q:l:rm:i", opts, "fork-vm", 1) {
+    case 'd':
+        debug = 1;
+        break;
+    case 'p':
+        pause = 1;
+        break;
+    case 'm':
+        max_vcpus = atoi(optarg);
+        break;
+    case 'C':
+        config_file = optarg;
+        break;
+    case 'Q':
+        dm_restore_file = optarg;
+        break;
+    case 'l':
+        if ( !strcmp(optarg, "no") )
+            launch_dm = 0;
+        if ( !strcmp(optarg, "yes") )
+            launch_dm = 1;
+        if ( !strcmp(optarg, "late") )
+            launch_dm = 2;
+        break;
+    case 'r':
+        reset = 1;
+        break;
+    case 'i':
+        allow_iommu = 1;
+        break;
+    default:
+        fprintf(stderr, "Unimplemented option(s)\n");
+        return EXIT_FAILURE;
+    }
+
+    if (argc-optind == 1) {
+        domid_in = atoi(argv[optind]);
+    } else {
+        help("fork-vm");
+        return EXIT_FAILURE;
+    }
+
+    if (launch_dm && (!config_file || !dm_restore_file)) {
+        fprintf(stderr, "Currently you must provide both -C and -Q options\n");
+        return EXIT_FAILURE;
+    }
+
+    if (reset) {
+        domid_out = domid_in;
+        if (libxl_domain_fork_reset(ctx, domid_in) == EXIT_FAILURE)
+            return EXIT_FAILURE;
+    }
+
+    if (launch_dm == 2 || reset) {
+        domid_out = domid_in;
+        rc = EXIT_SUCCESS;
+    } else {
+        if ( !max_vcpus )
+        {
+            fprintf(stderr, "Currently you must parent's max_vcpu for this option\n");
+            return EXIT_FAILURE;
+        }
+
+        rc = libxl_domain_fork_vm(ctx, domid_in, max_vcpus, allow_iommu, &domid_out);
+    }
+
+    if (rc == EXIT_SUCCESS) {
+        if ( launch_dm ) {
+            struct domain_create dom_info;
+            memset(&dom_info, 0, sizeof(dom_info));
+            dom_info.ddomid = domid_out;
+            dom_info.dm_restore_file = dm_restore_file;
+            dom_info.debug = debug;
+            dom_info.paused = pause;
+            dom_info.config_file = config_file;
+            dom_info.migrate_fd = -1;
+            dom_info.send_back_fd = -1;
+            rc = create_domain(&dom_info) < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
+        } else if ( !pause )
+            rc = libxl_domain_unpause(ctx, domid_out, NULL);
+    }
+
+    if (rc == EXIT_SUCCESS)
+        fprintf(stderr, "fork-vm command successfully returned domid: %u\n", domid_out);
+    else if ( domid_out != INVALID_DOMID )
+        libxl_domain_destroy(ctx, domid_out, 0);
+
+    return rc;
+}
+
+/*
+ * Local variables:
+ * mode: C
+ * c-basic-offset: 4
+ * indent-tabs-mode: nil
+ * End:
+ */
diff --git a/tools/xl/xl_vmcontrol.c b/tools/xl/xl_vmcontrol.c
index 17b4514c94..c64123d0a1 100644
--- a/tools/xl/xl_vmcontrol.c
+++ b/tools/xl/xl_vmcontrol.c
@@ -676,6 +676,12 @@  int create_domain(struct domain_create *dom_info)
 
     int restoring = (restore_file || (migrate_fd >= 0));
 
+#if defined(__i386__) || defined(__x86_64__)
+    /* VM forking */
+    uint32_t ddomid = dom_info->ddomid; // launch dm for this domain iff set
+    const char *dm_restore_file = dom_info->dm_restore_file;
+#endif
+
     libxl_domain_config_init(&d_config);
 
     if (restoring) {
@@ -928,6 +934,14 @@  start:
          * restore/migrate-receive it again.
          */
         restoring = 0;
+#if defined(__i386__) || defined(__x86_64__)
+    } else if ( ddomid ) {
+        d_config.dm_restore_file = dm_restore_file;
+        ret = libxl_domain_fork_launch_dm(ctx, &d_config, ddomid,
+                                          autoconnect_console_how);
+        domid = ddomid;
+        ddomid = INVALID_DOMID;
+#endif
     } else if (domid_soft_reset != INVALID_DOMID) {
         /* Do soft reset. */
         ret = libxl_domain_soft_reset(ctx, &d_config, domid_soft_reset,