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To:     Jim Mattson <jmattson@google.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v3 7/9] KVM: VMX: Add guest physical address check in EPT
 violation and misconfig
Message-ID: <f40e5d23-88b6-01c0-60f9-5419dac703a2@redhat.com>
Date:   Fri, 23 Oct 2020 19:16:41 +0200
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On 23/10/20 18:59, Jim Mattson wrote:
>> The problem is that page fault error code bits cannot be reconstructed
>> from bits 0..2 of the EPT violation exit qualification, if bit 8 is
>> clear in the exit qualification (that is, if the access causing the EPT
>> violation is to a paging-structure entry).  In that case bits 0..2 refer
>> to the paging-structure access rather than to the final access.  In fact
>> advanced information is not available at all for paging-structure access
>> EPT violations.
>
> True, but the in-kernel emulator can only handle a very small subset
> of the available instructions.
> 
> If bit 8 is set in the exit qualification, we should use the advanced
> VM-exit information. If it's clear, we should just do a software page
> walk of the guest's x86 page tables.

The information that we need is _not_ that provided by the advanced
VM-exit information (or by a page walk).  If a page is neither writable
nor executable, the advanced information doesn't say if the injected #PF
should be a W=1 or a F=1 fault.  We need the information in bits 0..2 of
the exit qualification for the final access, which however is not
available for the paging-structure access.

If bit 8 is set, however, we need not use the emulator indeed, as the
W/F/U bits are all available from either the exit qualification or in
the SS access rights.  The access.flat test in kvm-unit-tests covers all
this, so it will be easy to check the theory.

Paolo

> The in-kernel emulator should
> only be used as a last resort on hardware that doesn't support the
> advanced VM-exit information for EPT violations.
>