linux-kernel - RE: [PATCH V5 1/2] perf ignore LBR and extra

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Date:	Mon, 14 Jul 2014 13:36:02 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [PATCH V5 1/2] perf ignore LBR and extra_regs



> >>> For reproducing the issue, please build the kernel with
> CONFIG_KVM_INTEL = y (for host kernel).
> >>> And CONFIG_PARAVIRT = n and CONFIG_KVM_GUEST = n (for guest
> kernel).
> >>
> >> I'm not sure this is a useful patch.
> >>
> >> This is #GP'ing just because of a limitation in the PMU; just compile
> >> the kernel with CONFIG_PARAVIRT
> >
> > How's that going to help? If you run kvm -host the VM is lying through
> > its teeth is the kernel is going to assume all those MSRs present,
> > PARAVIRT isn't going to help with this.
> >
> >> , or split the "rdmsr is always rdmsr_safe"
> >> behavior out of CONFIG_PARAVIRT and into a new Kconfig symbol.
> >
> > That's not useful either, because non of these code-paths are going to
> > check the return value.
> 
> Hmmm, I thought rdmsr_safe was going to return zero, but it just returns
> whatever happened to be in edx:eax which maybe should also be fixed.
> 
> Kan Liang, what happens if CONFIG_PARAVIRT=y?  Do you get garbage or
> just no events reported?
> 

Guest rdmsr/wrmsr will eventually call rdmsr_safe/wrmsr_safe. They will handle the #GP. So there is no error report in guest.


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