Re: [PATCH V5 1/2] perf ignore LBR and extra_regs

From: Paolo Bonzini
Date: Mon Jul 14 2014 - 08:41:23 EST

Il 14/07/2014 14:09, Peter Zijlstra ha scritto:
On Mon, Jul 14, 2014 at 01:55:03PM +0200, Paolo Bonzini wrote:
Il 10/07/2014 12:59, kan.liang@xxxxxxxxx ha scritto:
From: Kan Liang <kan.liang@xxxxxxxxx>

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

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?

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?

A first thing that could be done, is to have a way for the kernel to detect absence of LBR, for example an all-1s setting of the LBR format field of IA32_PERF_CAPABILITIES. If Intel can tell us "all 1s will never be used", we can have KVM set the field that way. The kernel then should disable LBR.

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