diff mbox

[v3,1/2] KVM: kvm-io: support cookies

Message ID 1372859439-18733-2-git-send-email-cornelia.huck@de.ibm.com (mailing list archive)
State New, archived
Headers show

Commit Message

Cornelia Huck July 3, 2013, 1:50 p.m. UTC
Add new functions kvm_io_bus_{read,write}_cookie() that allows users of
the kvm io infrastructure to use a cookie value to speed up lookup of a
device on an io bus.

kvm_io_bus_{read,write} now returns the index on the bus; existing callers
have been fixed up to accept return codes > 0.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
---
 arch/ia64/kvm/kvm-ia64.c   |   2 +-
 arch/powerpc/kvm/powerpc.c |   4 +-
 arch/x86/kvm/x86.c         |   6 +--
 include/linux/kvm_host.h   |   4 ++
 virt/kvm/kvm_main.c        | 102 ++++++++++++++++++++++++++++++++++++++-------
 5 files changed, 97 insertions(+), 21 deletions(-)

Comments

Gleb Natapov July 3, 2013, 1:56 p.m. UTC | #1
On Wed, Jul 03, 2013 at 03:50:38PM +0200, Cornelia Huck wrote:
> Add new functions kvm_io_bus_{read,write}_cookie() that allows users of
> the kvm io infrastructure to use a cookie value to speed up lookup of a
> device on an io bus.
> 
> kvm_io_bus_{read,write} now returns the index on the bus; existing callers
> have been fixed up to accept return codes > 0.
> 
> Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> ---
>  arch/ia64/kvm/kvm-ia64.c   |   2 +-
>  arch/powerpc/kvm/powerpc.c |   4 +-
>  arch/x86/kvm/x86.c         |   6 +--
>  include/linux/kvm_host.h   |   4 ++
>  virt/kvm/kvm_main.c        | 102 ++++++++++++++++++++++++++++++++++++++-------
>  5 files changed, 97 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/ia64/kvm/kvm-ia64.c b/arch/ia64/kvm/kvm-ia64.c
> index 5b2dc0d..465ab54 100644
> --- a/arch/ia64/kvm/kvm-ia64.c
> +++ b/arch/ia64/kvm/kvm-ia64.c
> @@ -250,7 +250,7 @@ mmio:
>  	else
>  		r = kvm_io_bus_write(vcpu->kvm, KVM_MMIO_BUS, p->addr,
>  				     p->size, &p->data);
> -	if (r)
> +	if (r < 0)
No need to change kvm_io_bus_(read|write) return value now. Just do
there:

  r = __kvm_io_bus_write(bus, &range, val);
  return  r >=0 ? 0 : r;

>  		printk(KERN_ERR"kvm: No iodevice found! addr:%lx\n", p->addr);
>  	p->state = STATE_IORESP_READY;
>  
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index 6316ee3..26c44d0 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -649,7 +649,7 @@ int kvmppc_handle_load(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  
>  	srcu_read_unlock(&vcpu->kvm->srcu, idx);
>  
> -	if (!ret) {
> +	if (ret >= 0) {
>  		kvmppc_complete_mmio_load(vcpu, run);
>  		vcpu->mmio_needed = 0;
>  		return EMULATE_DONE;
> @@ -711,7 +711,7 @@ int kvmppc_handle_store(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  
>  	srcu_read_unlock(&vcpu->kvm->srcu, idx);
>  
> -	if (!ret) {
> +	if (ret >= 0) {
>  		vcpu->mmio_needed = 0;
>  		return EMULATE_DONE;
>  	}
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 7d71c0f..d58744b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3860,7 +3860,7 @@ static int vcpu_mmio_write(struct kvm_vcpu *vcpu, gpa_t addr, int len,
>  		n = min(len, 8);
>  		if (!(vcpu->arch.apic &&
>  		      !kvm_iodevice_write(&vcpu->arch.apic->dev, addr, n, v))
> -		    && kvm_io_bus_write(vcpu->kvm, KVM_MMIO_BUS, addr, n, v))
> +		    && (kvm_io_bus_write(vcpu->kvm, KVM_MMIO_BUS, addr, n, v) < 0))
>  			break;
>  		handled += n;
>  		addr += n;
> @@ -3880,7 +3880,7 @@ static int vcpu_mmio_read(struct kvm_vcpu *vcpu, gpa_t addr, int len, void *v)
>  		n = min(len, 8);
>  		if (!(vcpu->arch.apic &&
>  		      !kvm_iodevice_read(&vcpu->arch.apic->dev, addr, n, v))
> -		    && kvm_io_bus_read(vcpu->kvm, KVM_MMIO_BUS, addr, n, v))
> +		    && (kvm_io_bus_read(vcpu->kvm, KVM_MMIO_BUS, addr, n, v) < 0))
>  			break;
>  		trace_kvm_mmio(KVM_TRACE_MMIO_READ, n, addr, *(u64 *)v);
>  		handled += n;
> @@ -4361,7 +4361,7 @@ static int kernel_pio(struct kvm_vcpu *vcpu, void *pd)
>  		r = kvm_io_bus_write(vcpu->kvm, KVM_PIO_BUS,
>  				     vcpu->arch.pio.port, vcpu->arch.pio.size,
>  				     pd);
> -	return r;
> +	return (r >= 0) ? 0 : r;
>  }
>  
>  static int emulator_pio_in_out(struct kvm_vcpu *vcpu, int size,
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index e3aae6d..60e261c9 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -159,8 +159,12 @@ enum kvm_bus {
>  
>  int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
>  		     int len, const void *val);
> +int kvm_io_bus_write_cookie(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
> +			    int len, const void *val, long cookie);
>  int kvm_io_bus_read(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr, int len,
>  		    void *val);
> +int kvm_io_bus_read_cookie(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
> +			   int len, void *val, long cookie);
>  int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
>  			    int len, struct kvm_io_device *dev);
>  int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 1580dd4..eca3e00 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2863,11 +2863,30 @@ static int kvm_io_bus_get_first_dev(struct kvm_io_bus *bus,
>  	return off;
>  }
>  
> +static int __kvm_io_bus_write(struct kvm_io_bus *bus,
> +			      struct kvm_io_range *range, const void *val)
> +{
> +	int idx;
> +
> +	idx = kvm_io_bus_get_first_dev(bus, range->addr, range->len);
> +	if (idx < 0)
> +		return -EOPNOTSUPP;
> +
> +	while (idx < bus->dev_count &&
> +		kvm_io_bus_sort_cmp(range, &bus->range[idx]) == 0) {
> +		if (!kvm_iodevice_write(bus->range[idx].dev, range->addr,
> +					range->len, val))
> +			return idx;
> +		idx++;
> +	}
> +
> +	return -EOPNOTSUPP;
> +}
> +
>  /* kvm_io_bus_write - called under kvm->slots_lock */
>  int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
>  		     int len, const void *val)
>  {
> -	int idx;
>  	struct kvm_io_bus *bus;
>  	struct kvm_io_range range;
>  
> @@ -2877,14 +2896,51 @@ int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
>  	};
>  
>  	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
> -	idx = kvm_io_bus_get_first_dev(bus, addr, len);
> +	return __kvm_io_bus_write(bus, &range, val);
> +}
> +
> +/* kvm_io_bus_write_cookie - called under kvm->slots_lock */
> +int kvm_io_bus_write_cookie(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
> +			    int len, const void *val, long cookie)
> +{
> +	struct kvm_io_bus *bus;
> +	struct kvm_io_range range;
> +
> +	range = (struct kvm_io_range) {
> +		.addr = addr,
> +		.len = len,
> +	};
> +
> +	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
> +
> +	/* First try the device referenced by cookie. */
> +	if ((cookie >= 0) && (cookie < bus->dev_count) &&
> +	    (kvm_io_bus_sort_cmp(&range, &bus->range[cookie]) == 0))
> +		if (!kvm_iodevice_write(bus->range[cookie].dev, addr, len,
> +					val))
> +			return cookie;
> +
> +	/*
> +	 * cookie contained garbage; fall back to search and return the
> +	 * correct cookie value.
> +	 */
> +	return __kvm_io_bus_write(bus, &range, val);
> +}
> +
> +static int __kvm_io_bus_read(struct kvm_io_bus *bus, struct kvm_io_range *range,
> +			     void *val)
> +{
> +	int idx;
> +
> +	idx = kvm_io_bus_get_first_dev(bus, range->addr, range->len);
>  	if (idx < 0)
>  		return -EOPNOTSUPP;
>  
>  	while (idx < bus->dev_count &&
>  		kvm_io_bus_sort_cmp(&range, &bus->range[idx]) == 0) {
> -		if (!kvm_iodevice_write(bus->range[idx].dev, addr, len, val))
> -			return 0;
> +		if (!kvm_iodevice_read(bus->range[idx].dev, range->addr,
> +				       range->len, val))
> +			return idx;
>  		idx++;
>  	}
>  
> @@ -2895,7 +2951,6 @@ int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
>  int kvm_io_bus_read(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
>  		    int len, void *val)
>  {
> -	int idx;
>  	struct kvm_io_bus *bus;
>  	struct kvm_io_range range;
>  
> @@ -2905,18 +2960,35 @@ int kvm_io_bus_read(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
>  	};
>  
>  	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
> -	idx = kvm_io_bus_get_first_dev(bus, addr, len);
> -	if (idx < 0)
> -		return -EOPNOTSUPP;
> +	return __kvm_io_bus_read(bus, &range, val);
> +}
>  
> -	while (idx < bus->dev_count &&
> -		kvm_io_bus_sort_cmp(&range, &bus->range[idx]) == 0) {
> -		if (!kvm_iodevice_read(bus->range[idx].dev, addr, len, val))
> -			return 0;
> -		idx++;
> -	}
> +/* kvm_io_bus_read_cookie - called under kvm->slots_lock */
> +int kvm_io_bus_read_cookie(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
> +			   int len, void *val, long cookie)
> +{
> +	struct kvm_io_bus *bus;
> +	struct kvm_io_range range;
>  
> -	return -EOPNOTSUPP;
> +	range = (struct kvm_io_range) {
> +		.addr = addr,
> +		.len = len,
> +	};
> +
> +	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
> +
> +	/* First try the device referenced by cookie. */
> +	if ((cookie >= 0) && (cookie < bus->dev_count) &&
> +	    (kvm_io_bus_sort_cmp(&range, &bus->range[cookie]) == 0))
> +		if (!kvm_iodevice_read(bus->range[cookie].dev, addr, len,
> +				       val))
> +			return cookie;
> +
> +	/*
> +	 * cookie contained garbage; fall back to search and return the
> +	 * correct cookie value.
> +	 */
> +	return __kvm_io_bus_read(bus, &range, val);
>  }
>  
>  /* Caller must hold slots_lock. */
> -- 
> 1.8.2.2

--
			Gleb.
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Cornelia Huck July 3, 2013, 2:14 p.m. UTC | #2
On Wed, 3 Jul 2013 16:56:42 +0300
Gleb Natapov <gleb@redhat.com> wrote:

> On Wed, Jul 03, 2013 at 03:50:38PM +0200, Cornelia Huck wrote:
> > Add new functions kvm_io_bus_{read,write}_cookie() that allows users of
> > the kvm io infrastructure to use a cookie value to speed up lookup of a
> > device on an io bus.
> > 
> > kvm_io_bus_{read,write} now returns the index on the bus; existing callers
> > have been fixed up to accept return codes > 0.
> > 
> > Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> > ---
> >  arch/ia64/kvm/kvm-ia64.c   |   2 +-
> >  arch/powerpc/kvm/powerpc.c |   4 +-
> >  arch/x86/kvm/x86.c         |   6 +--
> >  include/linux/kvm_host.h   |   4 ++
> >  virt/kvm/kvm_main.c        | 102 ++++++++++++++++++++++++++++++++++++++-------
> >  5 files changed, 97 insertions(+), 21 deletions(-)
> > 
> > diff --git a/arch/ia64/kvm/kvm-ia64.c b/arch/ia64/kvm/kvm-ia64.c
> > index 5b2dc0d..465ab54 100644
> > --- a/arch/ia64/kvm/kvm-ia64.c
> > +++ b/arch/ia64/kvm/kvm-ia64.c
> > @@ -250,7 +250,7 @@ mmio:
> >  	else
> >  		r = kvm_io_bus_write(vcpu->kvm, KVM_MMIO_BUS, p->addr,
> >  				     p->size, &p->data);
> > -	if (r)
> > +	if (r < 0)
> No need to change kvm_io_bus_(read|write) return value now. Just do
> there:
> 
>   r = __kvm_io_bus_write(bus, &range, val);
>   return  r >=0 ? 0 : r;
> 

Well, if the index is not interesting for anything but the cookie code,
I can certainly drop it.

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Gleb Natapov July 3, 2013, 2:26 p.m. UTC | #3
On Wed, Jul 03, 2013 at 04:14:39PM +0200, Cornelia Huck wrote:
> On Wed, 3 Jul 2013 16:56:42 +0300
> Gleb Natapov <gleb@redhat.com> wrote:
> 
> > On Wed, Jul 03, 2013 at 03:50:38PM +0200, Cornelia Huck wrote:
> > > Add new functions kvm_io_bus_{read,write}_cookie() that allows users of
> > > the kvm io infrastructure to use a cookie value to speed up lookup of a
> > > device on an io bus.
> > > 
> > > kvm_io_bus_{read,write} now returns the index on the bus; existing callers
> > > have been fixed up to accept return codes > 0.
> > > 
> > > Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> > > ---
> > >  arch/ia64/kvm/kvm-ia64.c   |   2 +-
> > >  arch/powerpc/kvm/powerpc.c |   4 +-
> > >  arch/x86/kvm/x86.c         |   6 +--
> > >  include/linux/kvm_host.h   |   4 ++
> > >  virt/kvm/kvm_main.c        | 102 ++++++++++++++++++++++++++++++++++++++-------
> > >  5 files changed, 97 insertions(+), 21 deletions(-)
> > > 
> > > diff --git a/arch/ia64/kvm/kvm-ia64.c b/arch/ia64/kvm/kvm-ia64.c
> > > index 5b2dc0d..465ab54 100644
> > > --- a/arch/ia64/kvm/kvm-ia64.c
> > > +++ b/arch/ia64/kvm/kvm-ia64.c
> > > @@ -250,7 +250,7 @@ mmio:
> > >  	else
> > >  		r = kvm_io_bus_write(vcpu->kvm, KVM_MMIO_BUS, p->addr,
> > >  				     p->size, &p->data);
> > > -	if (r)
> > > +	if (r < 0)
> > No need to change kvm_io_bus_(read|write) return value now. Just do
> > there:
> > 
> >   r = __kvm_io_bus_write(bus, &range, val);
> >   return  r >=0 ? 0 : r;
> > 
> 
> Well, if the index is not interesting for anything but the cookie code,
> I can certainly drop it.
It is certainly not right now.

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diff mbox

Patch

diff --git a/arch/ia64/kvm/kvm-ia64.c b/arch/ia64/kvm/kvm-ia64.c
index 5b2dc0d..465ab54 100644
--- a/arch/ia64/kvm/kvm-ia64.c
+++ b/arch/ia64/kvm/kvm-ia64.c
@@ -250,7 +250,7 @@  mmio:
 	else
 		r = kvm_io_bus_write(vcpu->kvm, KVM_MMIO_BUS, p->addr,
 				     p->size, &p->data);
-	if (r)
+	if (r < 0)
 		printk(KERN_ERR"kvm: No iodevice found! addr:%lx\n", p->addr);
 	p->state = STATE_IORESP_READY;
 
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index 6316ee3..26c44d0 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -649,7 +649,7 @@  int kvmppc_handle_load(struct kvm_run *run, struct kvm_vcpu *vcpu,
 
 	srcu_read_unlock(&vcpu->kvm->srcu, idx);
 
-	if (!ret) {
+	if (ret >= 0) {
 		kvmppc_complete_mmio_load(vcpu, run);
 		vcpu->mmio_needed = 0;
 		return EMULATE_DONE;
@@ -711,7 +711,7 @@  int kvmppc_handle_store(struct kvm_run *run, struct kvm_vcpu *vcpu,
 
 	srcu_read_unlock(&vcpu->kvm->srcu, idx);
 
-	if (!ret) {
+	if (ret >= 0) {
 		vcpu->mmio_needed = 0;
 		return EMULATE_DONE;
 	}
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7d71c0f..d58744b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3860,7 +3860,7 @@  static int vcpu_mmio_write(struct kvm_vcpu *vcpu, gpa_t addr, int len,
 		n = min(len, 8);
 		if (!(vcpu->arch.apic &&
 		      !kvm_iodevice_write(&vcpu->arch.apic->dev, addr, n, v))
-		    && kvm_io_bus_write(vcpu->kvm, KVM_MMIO_BUS, addr, n, v))
+		    && (kvm_io_bus_write(vcpu->kvm, KVM_MMIO_BUS, addr, n, v) < 0))
 			break;
 		handled += n;
 		addr += n;
@@ -3880,7 +3880,7 @@  static int vcpu_mmio_read(struct kvm_vcpu *vcpu, gpa_t addr, int len, void *v)
 		n = min(len, 8);
 		if (!(vcpu->arch.apic &&
 		      !kvm_iodevice_read(&vcpu->arch.apic->dev, addr, n, v))
-		    && kvm_io_bus_read(vcpu->kvm, KVM_MMIO_BUS, addr, n, v))
+		    && (kvm_io_bus_read(vcpu->kvm, KVM_MMIO_BUS, addr, n, v) < 0))
 			break;
 		trace_kvm_mmio(KVM_TRACE_MMIO_READ, n, addr, *(u64 *)v);
 		handled += n;
@@ -4361,7 +4361,7 @@  static int kernel_pio(struct kvm_vcpu *vcpu, void *pd)
 		r = kvm_io_bus_write(vcpu->kvm, KVM_PIO_BUS,
 				     vcpu->arch.pio.port, vcpu->arch.pio.size,
 				     pd);
-	return r;
+	return (r >= 0) ? 0 : r;
 }
 
 static int emulator_pio_in_out(struct kvm_vcpu *vcpu, int size,
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index e3aae6d..60e261c9 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -159,8 +159,12 @@  enum kvm_bus {
 
 int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 		     int len, const void *val);
+int kvm_io_bus_write_cookie(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
+			    int len, const void *val, long cookie);
 int kvm_io_bus_read(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr, int len,
 		    void *val);
+int kvm_io_bus_read_cookie(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
+			   int len, void *val, long cookie);
 int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 			    int len, struct kvm_io_device *dev);
 int kvm_io_bus_unregister_dev(struct kvm *kvm, enum kvm_bus bus_idx,
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 1580dd4..eca3e00 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2863,11 +2863,30 @@  static int kvm_io_bus_get_first_dev(struct kvm_io_bus *bus,
 	return off;
 }
 
+static int __kvm_io_bus_write(struct kvm_io_bus *bus,
+			      struct kvm_io_range *range, const void *val)
+{
+	int idx;
+
+	idx = kvm_io_bus_get_first_dev(bus, range->addr, range->len);
+	if (idx < 0)
+		return -EOPNOTSUPP;
+
+	while (idx < bus->dev_count &&
+		kvm_io_bus_sort_cmp(range, &bus->range[idx]) == 0) {
+		if (!kvm_iodevice_write(bus->range[idx].dev, range->addr,
+					range->len, val))
+			return idx;
+		idx++;
+	}
+
+	return -EOPNOTSUPP;
+}
+
 /* kvm_io_bus_write - called under kvm->slots_lock */
 int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 		     int len, const void *val)
 {
-	int idx;
 	struct kvm_io_bus *bus;
 	struct kvm_io_range range;
 
@@ -2877,14 +2896,51 @@  int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 	};
 
 	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
-	idx = kvm_io_bus_get_first_dev(bus, addr, len);
+	return __kvm_io_bus_write(bus, &range, val);
+}
+
+/* kvm_io_bus_write_cookie - called under kvm->slots_lock */
+int kvm_io_bus_write_cookie(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
+			    int len, const void *val, long cookie)
+{
+	struct kvm_io_bus *bus;
+	struct kvm_io_range range;
+
+	range = (struct kvm_io_range) {
+		.addr = addr,
+		.len = len,
+	};
+
+	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
+
+	/* First try the device referenced by cookie. */
+	if ((cookie >= 0) && (cookie < bus->dev_count) &&
+	    (kvm_io_bus_sort_cmp(&range, &bus->range[cookie]) == 0))
+		if (!kvm_iodevice_write(bus->range[cookie].dev, addr, len,
+					val))
+			return cookie;
+
+	/*
+	 * cookie contained garbage; fall back to search and return the
+	 * correct cookie value.
+	 */
+	return __kvm_io_bus_write(bus, &range, val);
+}
+
+static int __kvm_io_bus_read(struct kvm_io_bus *bus, struct kvm_io_range *range,
+			     void *val)
+{
+	int idx;
+
+	idx = kvm_io_bus_get_first_dev(bus, range->addr, range->len);
 	if (idx < 0)
 		return -EOPNOTSUPP;
 
 	while (idx < bus->dev_count &&
 		kvm_io_bus_sort_cmp(&range, &bus->range[idx]) == 0) {
-		if (!kvm_iodevice_write(bus->range[idx].dev, addr, len, val))
-			return 0;
+		if (!kvm_iodevice_read(bus->range[idx].dev, range->addr,
+				       range->len, val))
+			return idx;
 		idx++;
 	}
 
@@ -2895,7 +2951,6 @@  int kvm_io_bus_write(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 int kvm_io_bus_read(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 		    int len, void *val)
 {
-	int idx;
 	struct kvm_io_bus *bus;
 	struct kvm_io_range range;
 
@@ -2905,18 +2960,35 @@  int kvm_io_bus_read(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 	};
 
 	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
-	idx = kvm_io_bus_get_first_dev(bus, addr, len);
-	if (idx < 0)
-		return -EOPNOTSUPP;
+	return __kvm_io_bus_read(bus, &range, val);
+}
 
-	while (idx < bus->dev_count &&
-		kvm_io_bus_sort_cmp(&range, &bus->range[idx]) == 0) {
-		if (!kvm_iodevice_read(bus->range[idx].dev, addr, len, val))
-			return 0;
-		idx++;
-	}
+/* kvm_io_bus_read_cookie - called under kvm->slots_lock */
+int kvm_io_bus_read_cookie(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
+			   int len, void *val, long cookie)
+{
+	struct kvm_io_bus *bus;
+	struct kvm_io_range range;
 
-	return -EOPNOTSUPP;
+	range = (struct kvm_io_range) {
+		.addr = addr,
+		.len = len,
+	};
+
+	bus = srcu_dereference(kvm->buses[bus_idx], &kvm->srcu);
+
+	/* First try the device referenced by cookie. */
+	if ((cookie >= 0) && (cookie < bus->dev_count) &&
+	    (kvm_io_bus_sort_cmp(&range, &bus->range[cookie]) == 0))
+		if (!kvm_iodevice_read(bus->range[cookie].dev, addr, len,
+				       val))
+			return cookie;
+
+	/*
+	 * cookie contained garbage; fall back to search and return the
+	 * correct cookie value.
+	 */
+	return __kvm_io_bus_read(bus, &range, val);
 }
 
 /* Caller must hold slots_lock. */