linux-kernel - Re: [PATCH v2] KVM: x86/MMU: Do not check unsync status for root SP.

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Message-ID: <6ca2d73c-703a-9964-48ae-e3d910bebc48@redhat.com>
Date:   Tue, 9 Feb 2021 08:46:42 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>
Cc:     seanjc@...gle.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org
Subject: Re: [PATCH v2] KVM: x86/MMU: Do not check unsync status for root SP.

On 09/02/21 04:33, Yu Zhang wrote:
> On Mon, Feb 08, 2021 at 05:47:22PM +0100, Paolo Bonzini wrote:
>> On 08/02/21 14:49, Yu Zhang wrote:
>>> On Mon, Feb 08, 2021 at 12:36:57PM +0100, Paolo Bonzini wrote:
>>>> On 07/02/21 13:22, Yu Zhang wrote:
>>>>> In shadow page table, only leaf SPs may be marked as unsync.
>>>>> And for non-leaf SPs, we use unsync_children to keep the number
>>>>> of the unsynced children. In kvm_mmu_sync_root(), sp->unsync
>>>>> shall always be zero for the root SP, , hence no need to check
>>>>> it. Instead, a warning inside mmu_sync_children() is added, in
>>>>> case someone incorrectly used it.
>>>>>
>>>>> Also, clarify the mmu_need_write_protect(), by moving the warning
>>>>> into kvm_unsync_page().
>>>>>
>>>>> Signed-off-by: Yu Zhang <yu.c.zhang@...ux.intel.com>
>>>>> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>>>>
>>>> This should really be more of a Co-developed-by, and there are a couple
>>>> adjustments that could be made in the commit message.  I've queued the patch
>>>> and I'll fix it up later.
>>>
>>> Indeed. Thanks for the remind, and I'll pay attention in the future. :)
>>
>> Also:
>>
>> arch/x86/kvm/mmu/mmu.c: In function ‘mmu_sync_children’:
>> arch/x86/kvm/mmu/mmu.c:2002:17: error: ‘sp’ is used uninitialized in this
>> function [-Werror=uninitialized]
>>    WARN_ON_ONCE(sp->unsync);
> 
> Oops. This is wrong. Should be WARN_ON_ONCE(parent->unsync);
> 
>>
>> so how was this tested?
>>
> 
> I ran access test in kvm-unit-test for previous version, which hasn't
> this code(also in my local repo "enable_ept" was explicitly set to
> 0 in order to test the shadow mode). But I did not test this one. I'm
> truely sorry for the negligence - even trying to compile should make
> this happen!
> 
> Should we submit another version? Any suggestions on the test cases?

Yes, please send v3.

The commit message can be:

In shadow page table, only leaf SPs may be marked as unsync; instead, 
for non-leaf SPs, we store the number of unsynced children in 
unsync_children.  Therefore, in kvm_mmu_sync_root(), sp->unsync
shall always be zero for the root SP and there is no need to check
it.  Remove the check, and add a warning inside mmu_sync_children() to 
assert that the flags are used properly.

While at it, move the warning from mmu_need_write_protect() to 
kvm_unsync_page().

Paolo