Date: Wed, 1 Aug 2012 17:34:43 +0000 (UTC) From: Konstantin Belousov <kib@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r238975 - head/sys/x86/x86 Message-ID: <201208011734.q71HYh3C080349@svn.freebsd.org>
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Author: kib Date: Wed Aug 1 17:34:43 2012 New Revision: 238975 URL: http://svn.freebsd.org/changeset/base/238975 Log: Do a trivial reformatting of the comment, to record the proper commit message for r238973: Rdtsc instruction is not synchronized, it seems on some Intel cores it can bypass even the locked instructions. As a result, rdtsc executed on different cores may return unordered TSC values even when the rdtsc appearance in the instruction sequences is provably ordered. Similarly to what has been done in r238755 for TSC synchronization test, add explicit fences right before rdtsc in the timecounters 'get' functions. Intel recommends to use LFENCE, while AMD refers to MFENCE. For VIA follow what Linux does and use LFENCE. With this change, I see no reordered reads of TSC on Nehalem. Change the rmb() to inlined CPUID in the SMP TSC synchronization test. On i386, locked instruction is used for rmb(), and as noted earlier, it is not enough. Since i386 machine may not support SSE2, do simplest possible synchronization with CPUID. MFC after: 1 week Discussed with: avg, bde, jkim Modified: head/sys/x86/x86/tsc.c Modified: head/sys/x86/x86/tsc.c ============================================================================== --- head/sys/x86/x86/tsc.c Wed Aug 1 17:31:31 2012 (r238974) +++ head/sys/x86/x86/tsc.c Wed Aug 1 17:34:43 2012 (r238975) @@ -354,8 +354,7 @@ init_TSC(void) * timecounters use MFENCE for AMD CPUs, and LFENCE for others (Intel * and VIA) when SSE2 is present, and nothing on older machines which * also do not issue RDTSC prematurely. There, testing for SSE2 and - * vendor is too cumbersome, and we learn about TSC presence from - * CPUID. + * vendor is too cumbersome, and we learn about TSC presence from CPUID. * * Do not use do_cpuid(), since we do not need CPUID results, which * have to be written into memory with do_cpuid().
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