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Date: Thu, 16 Feb 2017 18:50:58 +0100 (CET) From: Thomas Gleixner <tglx@...utronix.de> To: Vitaly Kuznetsov <vkuznets@...hat.com> cc: Andy Lutomirski <luto@...capital.net>, "K. Y. Srinivasan" <kys@...rosoft.com>, X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Haiyang Zhang <haiyangz@...rosoft.com>, Stephen Hemminger <sthemmin@...rosoft.com>, Dexuan Cui <decui@...rosoft.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, devel@...uxdriverproject.org, Linux Virtualization <virtualization@...ts.linux-foundation.org> Subject: Re: [PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource support On Wed, 15 Feb 2017, Vitaly Kuznetsov wrote: > Actually, we already have an implementation of TSC page update in KVM > (see arch/x86/kvm/hyperv.c, kvm_hv_setup_tsc_page()) and the update does > the following: > > 0) stash seq into seq_prev > 1) seq = 0 making all reads from the page invalid > 2) smp_wmb() > 3) update tsc_scale, tsc_offset > 4) smp_wmb() > 5) set seq = seq_prev + 1 I hope they handle the case where seq_prev overflows and becomes 0 :) > As far as I understand this helps with situations you described above as > guest will notice either invalid value of 0 or seq change. In case the > implementation in real Hyper-V is the same we're safe with compile > barriers only. On x86 that's correct. smp_rmb() resolves to barrier(), but you certainly need the smp_wmb() on the writer side. Now looking at the above your reader side code is bogus: + while (1) { + sequence = tsc_pg->tsc_sequence; + if (!sequence) + break; Why would you break out of the loop when seq is 0? The 0 is just telling you that there is an update in progress. The Linux seqcount writer side is: seq++; smp_wmb(); update... smp_wmb(); seq++; and it's defined that an odd sequence count, i.e. bit 0 set means update in progress. Which is nice, because you don't have to treat 0 special on the writer side and you don't need extra storage to stash seq away :) So the reader side does: do { while (1) { s = READ_ONCE(seq); if (!(s & 0x01)) break; cpu_relax(); } smp_rmb(); read data ... smp_rmb(); } while (s != seq) So for that hyperv thing you want: do { while (1) { s = READ_ONCE(seq); if (s) break; cpu_relax(); } smp_rmb(); read data ... smp_rmb(); } while (s != seq) Thanks, tglx
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