Re: [Xen-devel] [PATCH for-4.13] x86/tsc: update vcpu time info on guest TSC adjustments

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Oct 23, 2019 at 11:55:13AM +0100, Wei Liu wrote:
> On Wed, Oct 23, 2019 at 10:57:39AM +0200, Roger Pau Monne wrote:
> > If a HVM/PVH guest writes to MSR_IA32_TSC{_ADJUST} and thus changes
> > the value of the time stamp counter the vcpu time info must also be
> > updated, or the time calculated by the guest using the Xen PV clock
> > interface will be skewed.
> > 
> > Update the vcpu time info when the guest writes to either MSR_IA32_TSC
> > or MSR_IA32_TSC_ADJUST. This fixes lockups seen when running the
> > pv-shim on AMD hardware, since the shim will aggressively try to keep
> > TSCs in sync by periodically writing to MSR_IA32_TSC if the TSC is not
> > reliable.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> > Cc: Jürgen Groß <jgross@xxxxxxxx>
> > ---
> >  xen/arch/x86/hvm/hvm.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> > index 9acd359c99..e4c0425330 100644
> > --- a/xen/arch/x86/hvm/hvm.c
> > +++ b/xen/arch/x86/hvm/hvm.c
> > @@ -442,6 +442,8 @@ static void hvm_set_guest_tsc_msr(struct vcpu *v, u64 
> > guest_tsc)
> >  
> >      hvm_set_guest_tsc(v, guest_tsc);
> >      v->arch.hvm.msr_tsc_adjust += v->arch.hvm.cache_tsc_offset - 
> > tsc_offset;
> > +    if ( v == current )
> > +        update_vcpu_system_time(v);
> 
> Why not get rid of the test and always call update_vcpu_system_time(v)?

If the vcpu is not running the update will be done by the scheduling
code in schedule_tail. Calling update_vcpu_system_time unconditionally
would work, I was just trying to avoid the overhead if it's not really
needed.

Thanks, Roger.

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