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Date:	Mon, 14 Jul 2014 14:09:22 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	kan.liang@...el.com, andi@...stfloor.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH V5 1/2] perf ignore LBR and extra_regs

On Mon, Jul 14, 2014 at 01:55:03PM +0200, Paolo Bonzini wrote:
> Il 10/07/2014 12:59, kan.liang@...el.com ha scritto:
> >From: Kan Liang <kan.liang@...el.com>
> >
> >x86, perf: Protect LBR and extra_regs against KVM lying
> >
> >With -cpu host, KVM reports LBR and extra_regs support, if the host has support.
> >When the guest perf driver tries to access LBR or extra_regs MSR,
> >it #GPs all MSR accesses,since KVM doesn't handle LBR and extra_regs support.
> >So check the related MSRs access right once at initialization time to avoid the error access at runtime.
> >
> >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.

> In fact there's no reason why LBR cannot be virtualized (though it does need
> support from the processor), and it may even be possible to support
> OFFCORE_RSP_X in the KVM virtual PMU.

But its not, so something needs to be done, right?

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