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Date: Thu, 18 Feb 2016 13:11:33 -0800 From: Andy Lutomirski <luto@...nel.org> To: "Christopher S. Hall" <christopher.s.hall@...el.com>, tglx@...utronix.de, richardcochran@...il.com, mingo@...hat.com, john.stultz@...aro.org, hpa@...or.com, jeffrey.t.kirsher@...el.com Cc: x86@...nel.org, linux-kernel@...r.kernel.org, intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org, kevin.b.stanton@...el.com, kevin.j.clarke@...el.com Subject: Re: [PATCH v7 6/8] x86: tsc: Always Running Timer (ART) correlated clocksource On 02/12/2016 12:25 PM, Christopher S. Hall wrote: > On modern Intel systems TSC is derived from the new Always Running Timer > (ART). ART can be captured simultaneous to the capture of > audio and network device clocks, allowing a correlation between timebases > to be constructed. Upon capture, the driver converts the captured ART > value to the appropriate system clock using the correlated clocksource > mechanism. > > On systems that support ART a new CPUID leaf (0x15) returns parameters > “m” and “n” such that: > > TSC_value = (ART_value * m) / n + k [n >= 2] > > [k is an offset that can adjusted by a privileged agent. The > IA32_TSC_ADJUST MSR is an example of an interface to adjust k. > See 17.14.4 of the Intel SDM for more details] > > Signed-off-by: Christopher S. Hall <christopher.s.hall@...el.com> > [jstultz: Tweaked to fix build issue, also reworked math for > 64bit division on 32bit systems] > Signed-off-by: John Stultz <john.stultz@...aro.org> > --- > arch/x86/include/asm/cpufeature.h | 3 ++- > arch/x86/include/asm/tsc.h | 2 ++ > arch/x86/kernel/cpu/scattered.c | 1 + > arch/x86/kernel/tsc.c | 50 +++++++++++++++++++++++++++++++++++++++ > 4 files changed, 55 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h > index 7ad8c94..111b892 100644 > --- a/arch/x86/include/asm/cpufeature.h > +++ b/arch/x86/include/asm/cpufeature.h > @@ -85,7 +85,7 @@ > #define X86_FEATURE_P4 ( 3*32+ 7) /* "" P4 */ > #define X86_FEATURE_CONSTANT_TSC ( 3*32+ 8) /* TSC ticks at a constant rate */ > #define X86_FEATURE_UP ( 3*32+ 9) /* smp kernel running on up */ > -/* free, was #define X86_FEATURE_FXSAVE_LEAK ( 3*32+10) * "" FXSAVE leaks FOP/FIP/FOP */ > +#define X86_FEATURE_ART (3*32+10) /* Platform has always running timer (ART) */ > #define X86_FEATURE_ARCH_PERFMON ( 3*32+11) /* Intel Architectural PerfMon */ > #define X86_FEATURE_PEBS ( 3*32+12) /* Precise-Event Based Sampling */ > #define X86_FEATURE_BTS ( 3*32+13) /* Branch Trace Store */ > @@ -188,6 +188,7 @@ > > #define X86_FEATURE_CPB ( 7*32+ 2) /* AMD Core Performance Boost */ > #define X86_FEATURE_EPB ( 7*32+ 3) /* IA32_ENERGY_PERF_BIAS support */ > +#define X86_FEATURE_INVARIANT_TSC (7*32+4) /* Intel Invariant TSC */ How is this related to the rest of the patch? > +/* > + * Convert ART to TSC given numerator/denominator found in detect_art() > + */ > +struct system_counterval_t convert_art_to_tsc(cycle_t art) > +{ > + u64 tmp, res, rem; > + > + rem = do_div(art, art_to_tsc_denominator); > + > + res = art * art_to_tsc_numerator; > + tmp = rem * art_to_tsc_numerator; > + > + do_div(tmp, art_to_tsc_denominator); > + res += tmp; > + > + return (struct system_counterval_t) {.cs = art_related_clocksource, > + .cycles = res}; The SDM and the patch description both mention an offset "k". Shouldn't this code at least have a comment about how it deals with the k != 0 case? --Andy
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