Re: [Qemu-devel] [PATCH v1] kvm/x86: Hyper-V tsc page setup

[Roman Kagan]

On Wed, Jan 20, 2016 at 05:44:49PM +0300, Denis V. Lunev wrote:
> On 01/20/2016 05:05 PM, Paolo Bonzini wrote:
> >
> >On 19/01/2016 08:48, Denis V. Lunev wrote:
> >>>diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> >>>index 6877b4d7e..93c9e25 100644
> >>>--- a/include/linux/kvm_host.h
> >>>+++ b/include/linux/kvm_host.h
> >>>@@ -145,6 +145,7 @@ static inline bool is_error_page(struct page *page)
> >>>   #define KVM_REQ_HV_RESET          29
> >>>   #define KVM_REQ_HV_EXIT           30
> >>>   #define KVM_REQ_HV_STIMER         31
> >>>+#define KVM_REQ_HV_TSC_PAGE       32
> >>>     #define KVM_REQ_MAX               64
> >>ping
> >Applied with this change:
> >
> >diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> >index d7716c6e2752..047c275717d3 100644
> >--- a/arch/x86/kvm/hyperv.c
> >+++ b/arch/x86/kvm/hyperv.c
> >@@ -842,7 +842,7 @@ int kvm_hv_setup_tsc_page(struct kvm_vcpu *vcpu)
> >                tsc_khz, tsc, tsc_scale, tsc_offset);
> >     tsc_ref.tsc_sequence++;
> >-    if (tsc_ref.tsc_sequence == 0)
> >+    if (tsc_ref.tsc_sequence == 0xFFFFFFFF tsc_ref.tsc_sequence == 0)
> >             tsc_ref.tsc_sequence = 1;
> >     tsc_ref.tsc_scale = tsc_scale;
> >
> >and renumbering KVM_REQ_HV_TSC_PAGE from 32 to 31.
> >
> >Paolo
> the latter does not seem to be correct to me.
> MS spec has bug, see this thread:
>    https://lkml.org/lkml/2015/11/2/655
> and the change was Acked-by: K.Y.

What happens in the patched code is this: when the sequence counter
wraps around and hits an invalid value, skip to the next valid one.

I think Paolo wanted to be compatible not only with the de facto
implementation in Windows Hyper-V guests, but also with the Linux driver
which followed the buggy spec and thought 0xFFFFFFFF to be invalid.

So the change is fine (as long as there's || rather than a space there
;)

Thanks,
Roman.