linux-kernel - Re: [PATCH v3] KVM: Handle MSR_IA32_PERF

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Message-ID: <20160531131630.GB30721@potion>
Date:	Tue, 31 May 2016 15:16:31 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	kmeaw@...dex-team.ru
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	"Gabriel L. Somlo" <gsomlo@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"gleb@...nel.org" <gleb@...nel.org>
Subject: Re: [PATCH v3] KVM: Handle MSR_IA32_PERF_CTL

2016-05-31 13:06+0300, kmeaw@...dex-team.ru:
> 31.05.2016, 11:21, "Paolo Bonzini" <pbonzini@...hat.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@...dex-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