From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH v7 6/8] x86: tsc: Always Running Timer (ART) correlated clocksource Date: Thu, 18 Feb 2016 13:11:33 -0800 Message-ID: <56C63385.9050703@kernel.org> References: <1455308729-6280-1-git-send-email-christopher.s.hall@intel.com> <1455308729-6280-7-git-send-email-christopher.s.hall@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: x86@kernel.org, linux-kernel@vger.kernel.org, intel-wired-lan@lists.osuosl.org, netdev@vger.kernel.org, kevin.b.stanton@intel.com, kevin.j.clarke@intel.com To: "Christopher S. Hall" , tglx@linutronix.de, richardcochran@gmail.com, mingo@redhat.com, john.stultz@linaro.org, hpa@zytor.com, jeffrey.t.kirsher@intel.com Return-path: In-Reply-To: <1455308729-6280-7-git-send-email-christopher.s.hall@intel.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 02/12/2016 12:25 PM, Christopher S. Hall wrote: > On modern Intel systems TSC is derived from the new Always Running Ti= mer > (ART). ART can be captured simultaneous to the capture of > audio and network device clocks, allowing a correlation between timeb= ases > to be constructed. Upon capture, the driver converts the captured ART > value to the appropriate system clock using the correlated clocksourc= e > mechanism. > > On systems that support ART a new CPUID leaf (0x15) returns parameter= s > =E2=80=9Cm=E2=80=9D and =E2=80=9Cn=E2=80=9D such that: > > TSC_value =3D (ART_value * m) / n + k [n >=3D 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 > [jstultz: Tweaked to fix build issue, also reworked math for > 64bit division on 32bit systems] > Signed-off-by: John Stultz > --- > 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 const= ant 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 ti= mer (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 suppor= t */ > +#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_ar= t() > + */ > +struct system_counterval_t convert_art_to_tsc(cycle_t art) > +{ > + u64 tmp, res, rem; > + > + rem =3D do_div(art, art_to_tsc_denominator); > + > + res =3D art * art_to_tsc_numerator; > + tmp =3D rem * art_to_tsc_numerator; > + > + do_div(tmp, art_to_tsc_denominator); > + res +=3D tmp; > + > + return (struct system_counterval_t) {.cs =3D art_related_clocksourc= e, > + .cycles =3D res}; The SDM and the patch description both mention an offset "k". Shouldn'= t=20 this code at least have a comment about how it deals with the k !=3D 0 = case? --Andy