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    SubjectRe: [PATCH v3] KVM: Handle MSR_IA32_PERF_CTL
    2016-05-31 13:06+0300, kmeaw@yandex-team.ru:
    > 31.05.2016, 11:21, "Paolo Bonzini" <pbonzini@redhat.com>:
    > >>  2016-05-27 17:22+0200, Radim Krčmář:
    > >>  > (I wonder why MacOS X doesn't read IA32_PERF_STATUS, though.)
    > >>
    > >>  Oh, it maybe does ... we already emulate status and return 0x1000 in its
    > >>  bottom 16 bits. I have no idea what is that supposed to mean, but I
    > >>  think we should return 0x1000 in IA32_PERF_CTL then.
    > >
    > > It's 1000, not 0x1000 (instead, on real hardware the value is typically a
    > > multiple of 256). It was added for Darwin too.
    > >
    > > Returning different values is okay, because they are different on real
    > > hardware too:
    > >
    > > (sudo dd if=/dev/cpu/0/msr skip=$((0x198)) iflag=skip_bytes bs=8 count=1;
    > >  sudo dd if=/dev/cpu/0/msr skip=$((0x199)) iflag=skip_bytes bs=8 count=1) | od -tx8
    > > 0000000 00001f3900001100 0000000000001300
    > >         ^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^
    > >         PERF_STATUS PERF_CTL
    > >
    > > And perhaps if we returned non-zero values for PERF_CTL Darwin would try to
    > > write to it. So returning zero is fine, I think. There is no correct answer...
    >
    > Thank you. I have removed MSR_IA32_PERF_CTL from emulated_msrs[]. Returning
    > 1000 (0x3e8) for PERF_STATUS and 0 for PERF_CTL works fine with MacOS X.
    >
    > Just in case here are MSRs from i5-4460:
    > PERF_STATUS: 0000202800002100
    > PERF_CTL: 0000000000002200
    >
    > Chaning KVM's PERL_CTL from 0 to 0x2200 does not seem to interfere with MacOS X
    > boot process. It does not attempt to wrmsr into this register.
    >
    > Here is a refined version of the patch:
    > --
    >
    > From: Dmitry Bilunov <kmeaw@yandex-team.ru>
    >
    > Intel CPUs having Turbo Boost feature implement an MSR to provide a
    > control interface via rdmsr/wrmsr instructions. One could detect the
    > presence of this feature by issuing one of these instructions and
    > handling the #GP exception which is generated in case the referenced MSR
    > is not implemented by the CPU.
    >
    > KVM's vCPU model behaves exactly as a real CPU in this case by injecting
    > a fault when MSR_IA32_PERF_CTL is called (which KVM does not support).
    > However, some operating systems use this register during an early boot
    > stage in which their kernel is not capable of handling #GP correctly,
    > causing #DP and finally a triple fault effectively resetting the vCPU.
    >
    > This patch implements a dummy handler for MSR_IA32_PERF_CTL to avoid the
    > crashes.
    > ---

    The code looks good. Please resend with your signed-off-by and
    preserved writespace (tabs were converted to spaces),

    thanks.

    > arch/x86/kvm/x86.c | 1 +
    > 1 file changed, 1 insertion(+)
    >
    > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    > index c805cf4..d0a5b4b 100644
    > --- a/arch/x86/kvm/x86.c
    > +++ b/arch/x86/kvm/x86.c
    > @@ -2314,6 +2314,7 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
    > case MSR_AMD64_NB_CFG:
    > case MSR_FAM10H_MMIO_CONF_BASE:
    > case MSR_AMD64_BU_CFG2:
    > + case MSR_IA32_PERF_CTL:
    > msr_info->data = 0;
    > break;
    > case MSR_K7_EVNTSEL0 ... MSR_K7_EVNTSEL3:
    > --
    > 2.8.2

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