From a5ebe0ba3dff658c5286e8d5f20e4328f719d5a3 Mon Sep 17 00:00:00 2001 From: George Dunlap Date: Wed, 3 Apr 2013 15:46:28 +0100 Subject: [PATCH] perf/x86: Check all MSRs before passing hw check check_hw_exists() has a number of checks which go to two exit paths: msr_fail and bios_fail. Checks classified as msr_fail will cause check_hw_exists() to return false, causing the PMU not to be used; bios_fail checks will only cause a warning to be printed, but will return true. The problem is that if there are both msr failures and bios failures, and the routine hits a bios_fail check first, it will exit early and return true, not finishing the rest of the msr checks. If those msrs are in fact broken, it will cause them to be used erroneously. In the case of a Xen PV VM, the guest OS has read access to all the MSRs, but write access is white-listed to supported features. Writes to unsupported MSRs have no effect. The PMU MSRs are not (typically) supported, because they are expensive to save and restore on a VM context switch. One of the "msr_fail" checks is supposed to detect this circumstance (ether for Xen or KVM) and disable the harware PMU. However, on one of my AMD boxen, there is (apparently) a broken BIOS which triggers one of the bios_fail checks. In particular, MSR_K7_EVNTSEL0 has the ARCH_PERFMON_EVENTSEL_ENABLE bit set. The guest kernel detects this because it has read access to all MSRs, and causes it to skip the rest of the checks and try to use the non-existent hardware PMU. This minimally causes a lot of useless instruction emulation and Xen console spam; it may cause other issues with the watchdog as well. This changset causes check_hw_exists() to go through all of the msr checks, failing and returning false if any of them fail. This makes sure that a guest running under Xen without a virtual PMU will detect that there is no functioning PMU and not attempt to use it. This problem affects kernels as far back as 3.2, and should thus be considered for backport. Signed-off-by: George Dunlap Cc: Konrad Wilk Cc: Ian Campbell Cc: David Vrabel Cc: Andrew Cooper Link: http://lkml.kernel.org/r/1365000388-32448-1-git-send-email-george.dunlap@eu.citrix.com Signed-off-by: Ingo Molnar --- arch/x86/kernel/cpu/perf_event.c | 28 +++++++++++++++++----------- 1 file changed, 17 insertions(+), 11 deletions(-) diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c index 5ed7a4c5baf7..1025f3c99d20 100644 --- a/arch/x86/kernel/cpu/perf_event.c +++ b/arch/x86/kernel/cpu/perf_event.c @@ -180,8 +180,9 @@ static void release_pmc_hardware(void) {} static bool check_hw_exists(void) { - u64 val, val_new = ~0; - int i, reg, ret = 0; + u64 val, val_fail, val_new= ~0; + int i, reg, reg_fail, ret = 0; + int bios_fail = 0; /* * Check to see if the BIOS enabled any of the counters, if so @@ -192,8 +193,11 @@ static bool check_hw_exists(void) ret = rdmsrl_safe(reg, &val); if (ret) goto msr_fail; - if (val & ARCH_PERFMON_EVENTSEL_ENABLE) - goto bios_fail; + if (val & ARCH_PERFMON_EVENTSEL_ENABLE) { + bios_fail = 1; + val_fail = val; + reg_fail = reg; + } } if (x86_pmu.num_counters_fixed) { @@ -202,8 +206,11 @@ static bool check_hw_exists(void) if (ret) goto msr_fail; for (i = 0; i < x86_pmu.num_counters_fixed; i++) { - if (val & (0x03 << i*4)) - goto bios_fail; + if (val & (0x03 << i*4)) { + bios_fail = 1; + val_fail = val; + reg_fail = reg; + } } } @@ -221,14 +228,13 @@ static bool check_hw_exists(void) if (ret || val != val_new) goto msr_fail; - return true; - -bios_fail: /* * We still allow the PMU driver to operate: */ - printk(KERN_CONT "Broken BIOS detected, complain to your hardware vendor.\n"); - printk(KERN_ERR FW_BUG "the BIOS has corrupted hw-PMU resources (MSR %x is %Lx)\n", reg, val); + if (bios_fail) { + printk(KERN_CONT "Broken BIOS detected, complain to your hardware vendor.\n"); + printk(KERN_ERR FW_BUG "the BIOS has corrupted hw-PMU resources (MSR %x is %Lx)\n", reg_fail, val_fail); + } return true; -- 2.20.1