Re: [Xen-devel] [PATCH v2 6/6] x86: implement Hyper-V clock source

[Wei Liu <wl@xxxxxxx>]

On Wed, 18 Dec 2019 at 20:24, Michael Kelley <mikelley@xxxxxxxxxxxxx> wrote:
>
> From: Durrant, Paul <pdurrant@xxxxxxxxxx> Sent: Wednesday, December 18, 2019 
> 7:24 AM
>
> > > From: Wei Liu <wei.liu.xen@xxxxxxxxx> On Behalf Of Wei Liu
> > > Sent: 18 December 2019 14:43
>
> [snip]
>
> > > +
> > > +static inline uint64_t read_hyperv_timer(void)
> > > +{
> > > +    uint64_t scale, offset, ret, tsc;
> > > +    uint32_t seq;
> > > +    const struct ms_hyperv_tsc_page *tsc_page = hyperv_tsc;
> > > +
> > > +    do {
> > > +        seq = tsc_page->tsc_sequence;
> > > +
> > > +        /* Seq 0 is special. It means the TSC enlightenment is not
> > > +         * available at the moment. The reference time can only be
> > > +         * obtained from the Reference Counter MSR.
> > > +         */
> > > +        if ( seq == 0 )
> >
> > Older versions of the spec used to use 0xFFFFFFFF I think, although when I 
> > look again they
> > seem to have been retro-actively fixed. In any case I think you should 
> > treat both
> > 0xFFFFFFFF and 0 as invalid.
>
> FWIW, the 0xFFFFFFFF was just a bug in the spec.  Hyper-V implementations only
> set the value to 0 to indicate invalid.  The equivalent Linux code checks 
> only for 0.
>

Thanks for chiming in, Michael.

In that case I will submit a fix to change Xen's viridian code to
remove the wrong value there.

Wei.

> Michael

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