Avi noticed that we have to use guest's value for the version field,
instead of keeping track of it ourselves. If we don't do that,
the following situation can arise:
vcpu->arch.hv_clock.version is initialized to zero.
Guest reads version (result: 2)
Guest starts reading data
Live migration; vcpu->arch.hv_clock.version is zeroed
Steal time update; vcpu->arch.hv_clock.version += 2; write to guest
Guest continues reading data
Guest reads version (result: 2)
Signed-off-by: Glauber Costa <[email protected]>
CC: Avi Kivity <[email protected]>
---
arch/x86/kvm/x86.c | 9 ++++++---
1 files changed, 6 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c39ab4a..7fdc84a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1151,15 +1151,18 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
vcpu->last_guest_tsc = tsc_timestamp;
vcpu->hv_clock.flags = 0;
+ shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0);
+
/*
* The interface expects us to write an even number signaling that the
* update is finished. Since the guest won't see the intermediate
- * state, we just increase by 2 at the end.
+ * state, we just increase by 2 at the end. We use the guest's value as
+ * a basis to make it migration-safe.
*/
+ memcpy(&vcpu->hv_clock, shared_kaddr + vcpu->time_offset,
+ sizeof(vcpu->hv_clock));
vcpu->hv_clock.version += 2;
- shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0);
-
memcpy(shared_kaddr + vcpu->time_offset, &vcpu->hv_clock,
sizeof(vcpu->hv_clock));
--
1.7.2.3
On Wed, Feb 2, 2011 at 10:51 AM, Glauber Costa <[email protected]> wrote:
> Avi noticed that we have to use guest's value for the version field,
> instead of keeping track of it ourselves. If we don't do that,
> the following situation can arise:
>
> vcpu->arch.hv_clock.version is initialized to zero.
> Guest reads version (result: 2)
> Guest starts reading data
> Live migration; vcpu->arch.hv_clock.version is zeroed
> Steal time update; vcpu->arch.hv_clock.version += 2; write to guest
> Guest continues reading data
> Guest reads version (result: 2)
Please ignore it. I did test this patch but appearently used the wrong
module, and it tricked me
this version is obviously wrong.
>
> Signed-off-by: Glauber Costa <[email protected]>
> CC: Avi Kivity <[email protected]>
> ---
> arch/x86/kvm/x86.c | 9 ++++++---
> 1 files changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c39ab4a..7fdc84a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1151,15 +1151,18 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> vcpu->last_guest_tsc = tsc_timestamp;
> vcpu->hv_clock.flags = 0;
>
> + shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0);
> +
> /*
> * The interface expects us to write an even number signaling that the
> * update is finished. Since the guest won't see the intermediate
> - * state, we just increase by 2 at the end.
> + * state, we just increase by 2 at the end. We use the guest's value as
> + * a basis to make it migration-safe.
> */
> + memcpy(&vcpu->hv_clock, shared_kaddr + vcpu->time_offset,
> + sizeof(vcpu->hv_clock));
> vcpu->hv_clock.version += 2;
>
> - shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0);
> -
> memcpy(shared_kaddr + vcpu->time_offset, &vcpu->hv_clock,
> sizeof(vcpu->hv_clock));
>
> --
> 1.7.2.3
>
> --
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