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Re: [Xen-devel] [PATCH v2 6/6] x86: implement Hyper-V clock source
- To: "Durrant, Paul" <pdurrant@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen Development List <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Michael Kelley <mikelley@xxxxxxxxxxxxx>
- Date: Wed, 18 Dec 2019 20:24:31 +0000
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- Thread-topic: [PATCH v2 6/6] x86: implement Hyper-V clock source
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.
Michael
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