linux-kernel - Re: [PATCH 2/2] KVM: VMX: Ignore userspace MSR filters for x2APIC when APICV is enabled

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Message-ID: <bcb15eb1-8d3e-ff6d-d11f-667884584f1f@amazon.com>
Date:   Wed, 7 Oct 2020 16:01:59 +0200
From:   Alexander Graf <graf@...zon.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
CC:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Aaron Lewis <aaronlewis@...gle.com>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH 2/2] KVM: VMX: Ignore userspace MSR filters for x2APIC
 when APICV is enabled



On 05.10.20 21:55, Sean Christopherson wrote:
> 
> Rework the resetting of the MSR bitmap for x2APIC MSRs to ignore
> userspace filtering when APICV is enabled.  Allowing userspace to
> intercept reads to x2APIC MSRs when APICV is fully enabled for the guest
> simply can't work.   The LAPIC and thus virtual APIC is in-kernel and
> cannot be directly accessed by userspace.  If userspace wants to
> intercept x2APIC MSRs, then it should first disable APICV.
> 
> Opportunistically change the behavior to reset the full range of MSRs if
> and only if APICV is enabled for KVM.  The MSR bitmaps are initialized
> to intercept all reads and writes by default, and enable_apicv cannot be
> toggled after KVM is loaded.  I.e. if APICV is disabled, simply toggle
> the TPR MSR accordingly.
> 
> Note, this still allows userspace to intercept reads and writes to TPR,
> and writes to EOI and SELF_IPI.  It is at least plausible userspace
> interception could work for those registers, though it is still silly.
> 
> Cc: Alexander Graf <graf@...zon.com>
> Cc: Aaron Lewis <aaronlewis@...gle.com>
> Cc: Peter Xu <peterx@...hat.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>

I'm not opposed in general to leaving APICV handled registers out of the 
filtering logic. However, this really needs a note in the documentation 
then, no?


Alex



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