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Date: Fri, 24 May 2019 16:55:57 -0700 From: Fenghua Yu <fenghua.yu@...el.com> To: "Thomas Gleixner" <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>, "H Peter Anvin" <hpa@...or.com>, "Andy Lutomirski" <luto@...capital.net>, "Andrew Cooper" <andrew.cooper3@...rix.com>, "Ashok Raj" <ashok.raj@...el.com>, "Tony Luck" <tony.luck@...el.com>, "Ravi V Shankar" <ravi.v.shankar@...el.com> Cc: "linux-kernel" <linux-kernel@...r.kernel.org>, "x86" <x86@...nel.org>, Fenghua Yu <fenghua.yu@...el.com> Subject: [PATCH v3 0/5] x86/umwait: Enable user wait instructions Today, if an application needs to wait for a very short duration they have to have spinloops. Spinloops consume more power and continue to use execution resources that could hurt its thread siblings in a core with hyperthreads. New instructions umonitor, umwait and tpause allow a low power alternative waiting at the same time could improve the HT sibling perform while giving it any power headroom. These instructions can be used in both user space and kernel space. A new MSR IA32_UMWAIT_CONTROL allows kernel to set a time limit in TSC-quanta that prevents user applications from waiting for a long time. This allows applications to yield the CPU and the user application should consider using other alternatives to wait. The processor supports two levels of optimized states: a light-weight power/performance optimized state (C0.1 state) or an improved power/performance optimized state (C0.2 state with deeper power saving and higher exit latency). The above MSR can be used to restrict entry to C0.2 and then any request for C0.2 will revert to C0.1. This patch set covers feature discovery, provides initial values for the MSR, adds some sysfs control files for admin to tweak the values in the MSR if needed. The sysfs interface files are in /sys/devices/system/cpu/umwait_control/ GCC 9 enables intrinsics for the instructions. To use the instructions, user applications should include <immintrin.h> and be compiled with -mwaitpkg. Detailed information on the instructions, the MSR, and syntax of the intrinsics can be found in the latest Intel Architecture Instruction Set Extensions and Future Features Programming Reference and Intel 64 and IA-32 Architectures Software Developer's Manual. Changelog: v3: Address issues pointed out by Andy Lutomirski: - Change default umwait max time to 100k TSC cycles - Setting up MSR on BSP during resume suspend/hibernation - A few other naming and coding changes as suggested - Some security concerns of the user wait instructions are not issues of the patches and cannot be addressed in the patch set. They will be discussed on lkml. Plus: - Add ABI document entry for umwait control sysfs interfaces v2: - Address comments from Thomas Gleixner and Andy Lutomirski - Remove vDSO functions - Add sysfs control file for umwait max time v1: Based on comments from Thomas: - Change user APIs to vDSO functions - Changed sysfs per comments from Thomas. - Change patch descriptions etc Fenghua Yu (5): x86/cpufeatures: Enumerate user wait instructions x86/umwait: Initialize umwait control values x86/umwait: Add sysfs interface to control umwait C0.2 state x86/umwait: Add sysfs interface to control umwait maximum time x86/umwait: Document umwait control sysfs interfaces .../ABI/testing/sysfs-devices-system-cpu | 21 ++ arch/x86/include/asm/cpufeatures.h | 1 + arch/x86/include/asm/msr-index.h | 4 + arch/x86/power/Makefile | 1 + arch/x86/power/umwait.c | 179 ++++++++++++++++++ 5 files changed, 206 insertions(+) create mode 100644 arch/x86/power/umwait.c -- 2.19.1
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