Re: [Xen-devel] [PATCH] xen/time: fix system_time for vtsc=1 PV guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.04.16 at 15:29, <sstabellini@xxxxxxxxxx> wrote:
> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -784,7 +784,7 @@ static void __update_vcpu_system_time(struct vcpu *v, int 
> force)
>      struct cpu_time       *t;
>      struct vcpu_time_info *u, _u = {};
>      struct domain *d = v->domain;
> -    s_time_t tsc_stamp;
> +    s_time_t stime_stamp, tsc_stamp = 0;

I don't see why the initializer needs adding here.

> @@ -807,7 +808,11 @@ static void __update_vcpu_system_time(struct vcpu *v, 
> int force)
>                  tsc_stamp = -gtime_to_gtsc(d, -stime);
>          }
>          else
> +        {
>              tsc_stamp = gtime_to_gtsc(d, stime);
> +            if (!tsc_stamp)

Coding style.

> +                stime_stamp = d->arch.vtsc_offset;
> +        }

While I can see this being the right thing for getting the two stamps
in sync, is that really helping the guest? Time ought to be not moving
forward until getting past vtsc_offset afaict, and that can't be good.
I.e. it would seem to me that it's gtime_to_gtsc() that needs
adjustment to properly deal with time < d->arch.vtsc_offset.

Plus I can't see why, in the worst case, the gTSC value can't be
wrapped through zero into negative (or really huge positive) range:
Such TSC values are certainly not invalid, and guests shouldn't really
make assumptions on TSC values being in the small positive range
when they boot.

Also, looking at all the involved code, I once again wonder whether
all the is_hvm_*() there shouldn't be has_hvm_container_*().

Jan


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