linux-kernel - Re: [PATCH 06/23] KVM: MMU: load new PGD once nested two-dimensional paging is initialized

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Date:   Mon, 7 Feb 2022 15:35:23 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        seanjc@...gle.com, vkuznets@...hat.com
Subject: Re: [PATCH 06/23] KVM: MMU: load new PGD once nested two-dimensional
 paging is initialized

On 2/7/22 14:50, Paolo Bonzini wrote:
>>>
>> Those checks are just for performance correct (to skip iterating through
>> the list of roots)?
>>
>> Either way, it's probably worth including a Fixes tag below.
> 
> There's no bug because __kvm_mmu_new_pgd is passed a correct new_role. 
> But it's unnecessarily complex as shown by patches 7 and 9.

root_level is not part of the role, but we're still safe because 
changing it requires to flip CR0.PG to 0 and back to 1.  This calls 
kvm_mmu_reset_context and unloads all roots, so it's not possible to 
have a cached PGD with the wrong root level.  On the first call the 
root_level might be incorrect, but then again there's no cached PGD.

(Also, shadow_root_level is either always or never >= PT64_ROOT_4LEVEL, 
so it's okay unless the guest_mmu was never set up and therefore no 
cached PGD exists).

Paolo