Messages in this thread | | | Date | Sat, 4 Mar 2017 10:07:07 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH v3 2/3] x86/hyperv: move TSC reading method to asm/mshyperv.h |
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On Fri, 3 Mar 2017, Stephen Hemminger wrote: > static inline u64 hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg) > { > u64 scale, offset, cur_tsc; > u32 start; > > /* > * The protocol for reading Hyper-V TSC page is specified in Hypervisor > * Top-Level Functional Specification ver. 3.0 and above. 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. The currently published specification > * versions (up to 4.0b) contain a mistake and wrongly claim '-1' > * instead of '0' as the special value, see commit c35b82ef0294. > * - ReferenceTime = > * ((RDTSC() * 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 { > start = READ_ONCE(tsc_pg->tsc_sequence); > smp_rmb(); > > if (unlikely(!start)) > return U64_MAX; > > scale = tsc_pg->tsc_scale; > offset = tsc_pg->tsc_offset; > > /* > * Make sure we read sequence after we read all other values > * from TSC page. > */ > smp_rmb(); > } while (unlikely(READ_ONCE(tsc_pg->tsc_sequence != start))); > > cur_tsc = rdtsc_ordered();
That's wrong. You need to read the TSC value together with the scale and offset. That's needs to be "atomic". You can only do the mult/shift outside.
> return mul_u64_u64_shr(cur_tsc, scale, 64) + offset; > }
Thanks,
tglx
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