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

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Date:	Mon, 14 Jul 2014 13:55:03 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	kan.liang@...el.com, peterz@...radead.org
CC:	andi@...stfloor.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH V5 1/2] perf ignore LBR and extra_regs

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, or split the "rdmsr is always 
rdmsr_safe" behavior out of CONFIG_PARAVIRT and into a new Kconfig symbol.

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.

Paolo
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