Re: [PATCH v3] KVM: Handle MSR_IA32_PERF_CTL
From: Radim KrÄmÃÅ
Date: Tue May 31 2016 - 09:16:42 EST
2016-05-31 13:06+0300, kmeaw@xxxxxxxxxxxxxx:
> 31.05.2016, 11:21, "Paolo Bonzini" <pbonzini@xxxxxxxxxx>:
> >> Â2016-05-27 17:22+0200, Radim KrÄmÃÅ:
> >> Â> (I wonder why MacOS X doesn't read IA32_PERF_STATUS, though.)
> >> ÂOh, it maybe does ... we already emulate status and return 0x1000 in its
> >> Âbottom 16 bits. I have no idea what is that supposed to mean, but I
> >> Âthink 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=$((0x198)) iflag=skip_bytes bs=8 count=1;
> > Âsudo dd if=/dev/cpu/0/msr skip=$((0x199)) iflag=skip_bytes bs=8 count=1) | od -tx8
> > 0000000 00001f3900001100 0000000000001300
> > ÂÂÂÂÂÂÂÂ^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^
> > ÂÂÂÂÂÂÂÂPERF_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@xxxxxxxxxxxxxx>
> 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
The code looks good. Please resend with your signed-off-by and
preserved writespace (tabs were converted to spaces),
> 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;
> case MSR_K7_EVNTSEL0 ... MSR_K7_EVNTSEL3: