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

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Message-ID: <438d42de-78e1-0ce9-6a06-38194de4abd4@redhat.com>
Date:   Sat, 11 Dec 2021 03:15:25 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.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 12/10/21 20:25, Jim Mattson wrote:
> 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)?

What's the rationale for guarding this with a capability check?  IIRC 
you don't have such checks for perf_event_open (apart for getting kernel 
addresses, which is not a problem for virtualization).

Paolo