diff mbox series

[v14,09/10] doc: add ptp_kvm introduction for arm64 support

Message ID 20200904092744.167655-10-jianyong.wu@arm.com
State New, archived
Headers show
Series Enable ptp_kvm for arm64 | expand

Commit Message

Jianyong Wu Sept. 4, 2020, 9:27 a.m. UTC
ptp_kvm implementation depends on hypercall using SMCCC. So we
introduce a new SMCCC service ID. This doc explain how we define
and use this new ID.

Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
---
 Documentation/virt/kvm/arm/ptp_kvm.rst | 72 ++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)
 create mode 100644 Documentation/virt/kvm/arm/ptp_kvm.rst

Comments

Marc Zyngier Sept. 4, 2020, 4:18 p.m. UTC | #1
On Fri, 04 Sep 2020 10:27:43 +0100,
Jianyong Wu <jianyong.wu@arm.com> wrote:
> 
> ptp_kvm implementation depends on hypercall using SMCCC. So we
> introduce a new SMCCC service ID. This doc explain how we define
> and use this new ID.
> 
> Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
> ---
>  Documentation/virt/kvm/arm/ptp_kvm.rst | 72 ++++++++++++++++++++++++++
>  1 file changed, 72 insertions(+)
>  create mode 100644 Documentation/virt/kvm/arm/ptp_kvm.rst
> 
> diff --git a/Documentation/virt/kvm/arm/ptp_kvm.rst b/Documentation/virt/kvm/arm/ptp_kvm.rst
> new file mode 100644
> index 000000000000..455591e2587a
> --- /dev/null
> +++ b/Documentation/virt/kvm/arm/ptp_kvm.rst
> @@ -0,0 +1,72 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +PTP_KVM support for arm64
> +=========================
> +
> +PTP_KVM is used for time sync between guest and host in a high precison.
> +It needs get wall time and counter value from host and transfer these data
> +to guest via hypercall service. So one more hypercall service should be
> +added.
> +
> +This new SMCCC hypercall will be defined as:
> +
> +* ARM_SMCCC_HYP_KVM_PTP_FUNC_ID: 0xC6000001
> +
> +As we only support 64-bits ptp_kvm client, so we choose SMC64/HVC64
> +calling convention.

This isn't what the code does, as it is explicitly set as an SMC32
service... Furthermore, we still run 32bit guests, and will do for the
foreseeable future. Having removed KVM support for 32bit doesn't mean
32bits are gone.

	M.
Jianyong Wu Sept. 7, 2020, 7:55 a.m. UTC | #2
Hi Marc,

> -----Original Message-----
> From: Marc Zyngier <maz@kernel.org>
> Sent: Saturday, September 5, 2020 12:19 AM
> To: Jianyong Wu <Jianyong.Wu@arm.com>
> Cc: netdev@vger.kernel.org; yangbo.lu@nxp.com; john.stultz@linaro.org;
> tglx@linutronix.de; pbonzini@redhat.com; sean.j.christopherson@intel.com;
> richardcochran@gmail.com; Mark Rutland <Mark.Rutland@arm.com>;
> will@kernel.org; Suzuki Poulose <Suzuki.Poulose@arm.com>; Steven Price
> <Steven.Price@arm.com>; linux-kernel@vger.kernel.org; linux-arm-
> kernel@lists.infradead.org; kvmarm@lists.cs.columbia.edu;
> kvm@vger.kernel.org; Steve Capper <Steve.Capper@arm.com>; Justin He
> <Justin.He@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH v14 09/10] doc: add ptp_kvm introduction for arm64
> support
> 
> On Fri, 04 Sep 2020 10:27:43 +0100,
> Jianyong Wu <jianyong.wu@arm.com> wrote:
> >
> > ptp_kvm implementation depends on hypercall using SMCCC. So we
> > introduce a new SMCCC service ID. This doc explain how we define and
> > use this new ID.
> >
> > Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
> > ---
> >  Documentation/virt/kvm/arm/ptp_kvm.rst | 72
> > ++++++++++++++++++++++++++
> >  1 file changed, 72 insertions(+)
> >  create mode 100644 Documentation/virt/kvm/arm/ptp_kvm.rst
> >
> > diff --git a/Documentation/virt/kvm/arm/ptp_kvm.rst
> > b/Documentation/virt/kvm/arm/ptp_kvm.rst
> > new file mode 100644
> > index 000000000000..455591e2587a
> > --- /dev/null
> > +++ b/Documentation/virt/kvm/arm/ptp_kvm.rst
> > @@ -0,0 +1,72 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +PTP_KVM support for arm64
> > +=========================
> > +
> > +PTP_KVM is used for time sync between guest and host in a high precison.
> > +It needs get wall time and counter value from host and transfer these
> > +data to guest via hypercall service. So one more hypercall service
> > +should be added.
> > +
> > +This new SMCCC hypercall will be defined as:
> > +
> > +* ARM_SMCCC_HYP_KVM_PTP_FUNC_ID: 0xC6000001
> > +
> > +As we only support 64-bits ptp_kvm client, so we choose SMC64/HVC64
> > +calling convention.
> 
> This isn't what the code does, as it is explicitly set as an SMC32 service...
> Furthermore, we still run 32bit guests, and will do for the foreseeable future.
> Having removed KVM support for 32bit doesn't mean 32bits are gone.

Sorry to have removed arm32 support. It's worthy to add arm32 support in.
I will add it next time.

Thanks
Jianyong 

> 
> 	M.
> 
> --
> Without deviation from the norm, progress is not possible.
diff mbox series

Patch

diff --git a/Documentation/virt/kvm/arm/ptp_kvm.rst b/Documentation/virt/kvm/arm/ptp_kvm.rst
new file mode 100644
index 000000000000..455591e2587a
--- /dev/null
+++ b/Documentation/virt/kvm/arm/ptp_kvm.rst
@@ -0,0 +1,72 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+PTP_KVM support for arm64
+=========================
+
+PTP_KVM is used for time sync between guest and host in a high precison.
+It needs get wall time and counter value from host and transfer these data
+to guest via hypercall service. So one more hypercall service should be
+added.
+
+This new SMCCC hypercall will be defined as:
+
+* ARM_SMCCC_HYP_KVM_PTP_FUNC_ID: 0xC6000001
+
+As we only support 64-bits ptp_kvm client, so we choose SMC64/HVC64
+calling convention.
+
+ARM_SMCCC_HYP_KVM_PTP_FUNC_ID:
+
+	============    ========    ==========
+	Function ID:    (uint32)    0xC6000001
+	Arguments:	(uint32)    ARM_PTP_PHY_COUNTER(1) or ARM_PTP_VIRT_COUNTER(0)
+				    which indicate acquiring physical counter or
+				    virtual counter respectively.
+	return value:   (uint64)    NOT_SUPPORTED (-1) or two values of wall clock
+				    time and counter cycle.
+	============    ========    ==========
+
+Why we need PTP_KVM?
+Currently, we offen use ntp (sync time with remote network clock) to sync time
+in VM. But the precision of ntp is subject to network delay so it's difficult
+to sync time in a high precision.
+
+kvm virtual ptp clock (ptp_kvm) offers another way to sync time in VM, in which
+the remote clock locates in the host instead of remote network clock. It
+targets to sync time between guest and host in virtualization environment and
+in this way, we can also keep the time of all the VMs running in the same host
+in sync. In general, the delay of communication between host and guest is quiet
+small, so ptp_kvm can offer time sync precision up to in order of nanosecond.
+Please keep in mind that ptp_kvm just limits itself to be a channel which
+transmit the remote clock from host to guest and leaves the time sync jobs to
+an application, eg. chrony, in usersapce of VM.
+
+How PTP_KVM works on arm64?
+After ptp_kvm initialized, there will be a new device node under /dev called
+ptp%d. A guest userspace service, like chrony, can use this device to get host
+walltime, sometimes also counter cycle, which depends on the service it calls.
+Then this guest userspace service can use those data to do the time sync for
+guest.
+Here is a rough sketch to show how kvm ptp clock works.
+
+|----------------------------|              |--------------------------|
+|       guest userspace      |              |          host            |
+|ioctl -> /dev/ptp%d         |              |                          |
+|       ^   |                |              |                          |
+|----------------------------|              |                          |
+|       |   | guest kernel   |              |                          |
+|       |   V      (get host walltime/counter cycle)                   |
+|      ptp_kvm -> hypercall - - - - - - - - - - ->hypercall service    |
+|                         <- - - - - - - - - - - -                     |
+|----------------------------|              |--------------------------|
+
+1. time sync service in guest userspace call ptp device through /dev/ptp%d.
+2. ptp_kvm module in guest recive this request then invoke hypercall to
+   route into host kernel to request host walltime/counter cycle.
+3. ptp_kvm hypercall service in host response to the request and send data
+   back.
+4. ptp (not ptp_kvm, ptp_kvm implemented on it and not shown above) in guest
+   copy the data to userspace.
+
+This ptp_kvm implementation focuses itself to step 2 and 3 and step 2 works
+in guest comparing step 3 works in host kernel.