Re: [Xen-devel] [PATCH] x86/vpmu_intel: Fix hypervisor crash by catching wrmsr fault

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

>
> Also, from the description in the SDM, PC flag bit it seems very
> disruptive to me.
> SDM says that if the bit is set then the processor toggles the PMi pin
> (generating a performance monitoring interrupt?)
> every time the event occurs. So if we program the counter to count
> "unhaulted core cycles", and set PC flag bit
> it will generate an interrupts every cycle!?

I checked how Linux treats this bit and there is this interesting commit:


commit a7b9d2ccc3d86303ee9314612d301966e04011c7
Author: Gleb Natapov <gleb@xxxxxxxxxx>
Date:   Sun Feb 26 16:55:40 2012 +0200

    KVM: PMU: warn when pin control is set in eventsel msr
   
    Print warning once if pin control bit is set in eventsel msr since
    emulation does not support it yet.
   
    Signed-off-by: Gleb Natapov <gleb@xxxxxxxxxx>
    Signed-off-by: Avi Kivity <avi@xxxxxxxxxx>

diff --git a/arch/x86/include/asm/perf_event.h
b/arch/x86/include/asm/perf_event.h
index 096c975..f1f7182 100644
--- a/arch/x86/include/asm/perf_event.h
+++ b/arch/x86/include/asm/perf_event.h
@@ -23,6 +23,7 @@
 #define ARCH_PERFMON_EVENTSEL_USR                      (1ULL << 16)
 #define ARCH_PERFMON_EVENTSEL_OS                       (1ULL << 17)
 #define ARCH_PERFMON_EVENTSEL_EDGE                     (1ULL << 18)
+#define ARCH_PERFMON_EVENTSEL_PIN_CONTROL              (1ULL << 19)
 #define ARCH_PERFMON_EVENTSEL_INT                      (1ULL << 20)
 #define ARCH_PERFMON_EVENTSEL_ANY                      (1ULL << 21)
 #define ARCH_PERFMON_EVENTSEL_ENABLE                   (1ULL << 22)
diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 3e48c1d..6af9a54 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -210,6 +210,9 @@ static void reprogram_gp_counter(struct kvm_pmc
*pmc, u64 eventsel)
        unsigned config, type = PERF_TYPE_RAW;
        u8 event_select, unit_mask;
 
+       if (eventsel & ARCH_PERFMON_EVENTSEL_PIN_CONTROL)
+               printk_once("kvm pmu: pin control bit is ignored\n");
+
        pmc->eventsel = eventsel;
 
        stop_counter(pmc);



-boris

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