Re: [PATCH 1/4] arm64: hyperv: Add core Hyper-V include files

Vitaly Kuznetsov
2018-12-10T17:43:34Z

kys@linuxonhyperv.com writes:

> +
> +static inline u64 hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg,
> + u64 *cur_tsc)
> +{
> + u64 scale, offset;
> + u32 sequence;
> +
> + /*
> + * The protocol for reading Hyper-V TSC page is specified in Hypervisor
> + * Top-Level Functional Specification. To get the reference time we
> + * must do the following:
> + * - READ ReferenceTscSequence
> + * A special '0' value indicates the time source is unreliable and we
> + * need to use something else.
> + * - ReferenceTime =
> + * ((HWclock val) * ReferenceTscScale) >> 64) + ReferenceTscOffset
> + * - READ ReferenceTscSequence again. In case its value has changed
> + * since our first reading we need to discard ReferenceTime and repeat
> + * the whole sequence as the hypervisor was updating the page in
> + * between.
> + */
> + do {
> + sequence = READ_ONCE(tsc_pg->tsc_sequence);

(sorry if this was already discussed before)

Current x86 code doing this actually checks for '0' here (note the
comment about this special value above):

sequence = READ_ONCE(tsc_pg->tsc_sequence);
if (!sequence)
return U64_MAX;

Was it removed intentionally (and we need to fix the comment then)?

--
Vitaly