linux-kernel - Re: [PATCH v3 9/9] KVM: x86: SVM: VMX: Make GUEST_MAXPHYADDR < HOST

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Date:   Fri, 10 Jul 2020 19:40:45 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Mohammed Gamal <mgamal@...hat.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, vkuznets@...hat.com,
        sean.j.christopherson@...el.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Babu Moger <babu.moger@....com>
Subject: Re: [PATCH v3 9/9] KVM: x86: SVM: VMX: Make GUEST_MAXPHYADDR <
 HOST_MAXPHYADDR support configurable

On 10/07/20 17:48, Mohammed Gamal wrote:
> The reason behind including this patch is unexpected behaviour we see
> with NPT vmexit handling in AMD processor.
> 
> With previous patch ("KVM: SVM: Add guest physical address check in
> NPF/PF interception") we see the followning error multiple times in
> the 'access' test in kvm-unit-tests:
> 
>             test pte.p pte.36 pde.p: FAIL: pte 2000021 expected 2000001
>             Dump mapping: address: 0x123400000000
>             ------L4: 24c3027
>             ------L3: 24c4027
>             ------L2: 24c5021
>             ------L1: 1002000021
> 
> This shows that the PTE's accessed bit is apparently being set by
> the CPU hardware before the NPF vmexit. This completely handled by
> hardware and can not be fixed in software.
> 
> This patch introduces a workaround. We add a boolean variable:
> 'allow_smaller_maxphyaddr'
> Which is set individually by VMX and SVM init routines. On VMX it's
> always set to true, on SVM it's only set to true when NPT is not
> enabled.
> 
> We also add a new capability KVM_CAP_SMALLER_MAXPHYADDR which
> allows userspace to query if the underlying architecture would
> support GUEST_MAXPHYADDR < HOST_MAXPHYADDR and hence act accordingly
> (e.g. qemu can decide if it would ignore the -cpu ..,phys-bits=X)
> 
> CC: Tom Lendacky <thomas.lendacky@....com>
> CC: Babu Moger <babu.moger@....com>
> Signed-off-by: Mohammed Gamal <mgamal@...hat.com>

Slightly rewritten commit message:

    KVM: x86: Add a capability for GUEST_MAXPHYADDR < HOST_MAXPHYADDR support
    
    This patch adds a new capability KVM_CAP_SMALLER_MAXPHYADDR which
    allows userspace to query if the underlying architecture would
    support GUEST_MAXPHYADDR < HOST_MAXPHYADDR and hence act accordingly
    (e.g. qemu can decide if it should warn for -cpu ..,phys-bits=X)
    
    The complications in this patch are due to unexpected (but documented)
    behaviour we see with NPF vmexit handling in AMD processor.  If
    SVM is modified to add guest physical address checks in the NPF
    and guest #PF paths, we see the followning error multiple times in
    the 'access' test in kvm-unit-tests:
    
                test pte.p pte.36 pde.p: FAIL: pte 2000021 expected 2000001
                Dump mapping: address: 0x123400000000
                ------L4: 24c3027
                ------L3: 24c4027
                ------L2: 24c5021
                ------L1: 1002000021
    
    This is because the PTE's accessed bit is set by the CPU hardware before
    the NPF vmexit. This is handled completely by hardware and cannot be fixed
    in software.
    
    Therefore, availability of the new capability depends on a boolean variable
    allow_smaller_maxphyaddr which is set individually by VMX and SVM init
    routines. On VMX it's always set to true, on SVM it's only set to true
    when NPT is not enabled.
    
    CC: Tom Lendacky <thomas.lendacky@....com>
    CC: Babu Moger <babu.moger@....com>
    Signed-off-by: Mohammed Gamal <mgamal@...hat.com>
    Message-Id: <20200710154811.418214-10-mgamal@...hat.com>
    Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Paolo