Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method
From: Thomas Gleixner <hidden>
Date: 2017-02-10 11:34:34
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Possibly related (same subject, not in this thread)
- 2017-02-13 · Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method · Thomas Gleixner <hidden>
- 2017-02-13 · Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method · Andy Lutomirski <luto@amacapital.net>
- 2017-02-13 · RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method · Thomas Gleixner <hidden>
- 2017-02-13 · RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method · Dexuan Cui <decui@microsoft.com>
- 2017-02-10 · RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method · Thomas Gleixner <hidden>
On Fri, 10 Feb 2017, Vitaly Kuznetsov wrote:
Thomas Gleixner [off-list ref] writes:quoted
On Thu, 9 Feb 2017, Vitaly Kuznetsov wrote:quoted
+#ifdef CONFIG_HYPERV_TSCPAGE +static notrace u64 vread_hvclock(int *mode) +{ + const struct ms_hyperv_tsc_page *tsc_pg = + (const struct ms_hyperv_tsc_page *)&hvclock_page; + u64 sequence, scale, offset, current_tick, cur_tsc; + + while (1) { + sequence = READ_ONCE(tsc_pg->tsc_sequence); + if (!sequence) + break; + + scale = READ_ONCE(tsc_pg->tsc_scale); + offset = READ_ONCE(tsc_pg->tsc_offset); + rdtscll(cur_tsc); + + current_tick = mul_u64_u64_shr(cur_tsc, scale, 64) + offset; + + if (READ_ONCE(tsc_pg->tsc_sequence) == sequence) + return current_tick;That sequence stuff lacks still a sensible explanation. It's fundamentally different from the sequence counting we do in the kernel, so documentation for it is really required.Sure, do you think the following would do?
Yes
quoted hunk ↗ jump to hunk
diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c index 4af10b4..886b600 100644 --- a/arch/x86/entry/vdso/vclock_gettime.c +++ b/arch/x86/entry/vdso/vclock_gettime.c@@ -154,6 +154,22 @@ static notrace u64 vread_hvclock(int *mode) (const struct ms_hyperv_tsc_page *)&hvclock_page; u64 sequence, scale, offset, current_tick, cur_tsc; + /* + * 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. + */ while (1) { sequence = READ_ONCE(tsc_pg->tsc_sequence); if (!sequence)-- Vitaly