linux-kernel - Re: [PATCH] KVM: x86/svm: Add module param to control PMU virtualization

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Date:   Fri, 10 Dec 2021 11:25:16 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Like Xu <like.xu.linux@...il.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/svm: Add module param to control PMU virtualization

On Thu, Nov 18, 2021 at 5:25 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 11/17/21 09:03, Like Xu wrote:
> > From: Like Xu <likexu@...cent.com>
> >
> > For Intel, the guest PMU can be disabled via clearing the PMU CPUID.
> > For AMD, all hw implementations support the base set of four
> > performance counters, with current mainstream hardware indicating
> > the presence of two additional counters via X86_FEATURE_PERFCTR_CORE.
> >
> > In the virtualized world, the AMD guest driver may detect
> > the presence of at least one counter MSR. Most hypervisor
> > vendors would introduce a module param (like lbrv for svm)
> > to disable PMU for all guests.
> >
> > Another control proposal per-VM is to pass PMU disable information
> > via MSR_IA32_PERF_CAPABILITIES or one bit in CPUID Fn4000_00[FF:00].
> > Both of methods require some guest-side changes, so a module
> > parameter may not be sufficiently granular, but practical enough.
> >
> > Signed-off-by: Like Xu <likexu@...cent.com>
> > ---
Thanks for this patch. It saves us from upstreaming our equivalent patch.

In the long run, I'd like to be able to override this system-wide
setting on a per-VM basis, for VMs that I trust. (Of course, this
implies that I trust the userspace process as well.)

How would you feel if we were to add a kvm ioctl to override this
setting, for a particular VM, guarded by an appropriate permissions
check, like capable(CAP_SYS_ADMIN) or capable(CAP_SYS_MODULE)?