Re: [PATCH] KVM: x86/mmu: Calculate and check "full" mmu_role for nested MMU

Paolo Bonzini
2021-06-11T16:49:33Z

On 11/06/21 18:45, Sean Christopherson wrote:
>> Along which path though? I would have naively expected those to be driven
>> only by the context->root_level.
> The functional code is driven by context->root_level, but if KVM doesn't include
> the level in the mmu_role then it will fail to update context->root_level when
> L2 changes from 32-bit PAE to 64-bit. If all the CR0/CR4/EFER bits remain the
> same, only the level will differ. Without this patch, role.level is always '0'
> for the nested MMU.
>

Yes the problem is the

if (new_role.as_u64 == g_context->mmu_role.as_u64)
return;

and the patch lets you preserve the optimization instead of dropping it.
I was wondering if I was missing something else because of the "when
walking the guest page tables" remark.

Paolo