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Subject: Re: [PATCH] KVM: VMX: Forbid userspace MSR filters for x2APIC
To:     Alexander Graf <graf@amazon.de>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Aaron Lewis <aaronlewis@google.com>,
        Peter Xu <peterx@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>
References: <20201019170519.1855564-1-pbonzini@redhat.com>
 <618E2129-7AB5-4F0D-A6C9-E782937FE935@amazon.de>
 <c9dd6726-2783-2dfd-14d1-5cec6f69f051@redhat.com>
 <bce2aee1-bfac-0640-066b-068fa5f12cf8@amazon.de>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <6edd5e08-92c2-40ff-57be-37b92d1ca2bc@redhat.com>
Date:   Tue, 20 Oct 2020 12:34:46 +0200
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On 20/10/20 11:48, Alexander Graf wrote:
> 
>     count: 1,
>     default_allow: false,
>     ranges: [
>         {
>             flags: KVM_MSR_FILTER_READ,
>             nmsrs: 1,
>             base: MSR_EFER,
>             bitmap: { 1 },
>         },
>     ],
> }
> 
> That filter would set all x2apic registers to "deny", but would not be
> caught by the code above. Conversely, a range that explicitly allows
> x2apic ranges with default_allow=0 would be rejected by this patch.

Yes, but the idea is that x2apic registers are always allowed, even
overriding default_allow, and therefore it makes no sense to have them
in a range.  The patch is only making things fail early for userspace,
the policy is defined by Sean's patch.

Paolo