RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method
From: Stephen Hemminger via Virtualization <hidden>
Date: 2017-02-09 18:27:35
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Why not use existing seqlock's? -----Original Message----- From: Thomas Gleixner [mailto:tglx@linutronix.de] Sent: Thursday, February 9, 2017 9:08 AM To: Vitaly Kuznetsov <vkuznets@redhat.com> Cc: x86@kernel.org; Andy Lutomirski <luto@amacapital.net>; Ingo Molnar <mingo@redhat.com>; H. Peter Anvin <hpa@zytor.com>; KY Srinivasan <kys@microsoft.com>; Haiyang Zhang <haiyangz@microsoft.com>; Stephen Hemminger <redacted>; Dexuan Cui <decui@microsoft.com>; linux-kernel@vger.kernel.org; devel@linuxdriverproject.org; virtualization@lists.linux-foundation.org Subject: Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method On Thu, 9 Feb 2017, Vitaly Kuznetsov wrote:
quoted hunk
+#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. Thanks, tglx