Messages in this thread | | | Date | Fri, 19 Apr 2013 17:29:59 +0100 | From | George Dunlap <> | Subject | Re: [PATCH v3] perf: Check all MSRs before passing hw check |
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Any comments? it's been 2 weeks now.
Thanks, -George
On 03/04/13 15:46, George Dunlap wrote: > 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. > > v3: > - Save the register and value which failed, and print them once at the end. > v2: > - Print the warning when the event happens so the reg,val make sense > - But print it only for the first such instance > - Update changelog to include details of failing system > > Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com> > CC: Konrad Wilk <konrad.wilk@oracle.com> > CC: Thomas Gleixner <tglx@linutronix.de> > CC: "H. Peter Anvin" <hpa@zytor.com> > CC: Ingo Molnar <mingo@kernel.org> > CC: x86@kernel.org > CC: Ian Campbell <ian.campbell@citrix.com> > CC: David Vrabel <david.vrabel@citrix.com> > CC: Andrew Cooper <andrew.cooper3@citrix.com> > --- > arch/x86/kernel/cpu/perf_event.c | 30 ++++++++++++++++++------------ > 1 file changed, 18 insertions(+), 12 deletions(-) > > diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c > index 6774c17..2456bae 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; >
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