Re: [PATCH 16/15] KVM: X86: Update mmu->pdptrs only when it is changed

Lai Jiangshan
2021-12-08T09:34:17Z

On 2021/12/8 17:09, Paolo Bonzini wrote:
> On 12/8/21 00:43, Sean Christopherson wrote:
>> what guarantees the that PDPTRs in the VMCS are sync'd with
>> mmu->pdptrs?  I'm not saying they aren't, I just want the changelog
>> to prove that they are.
>
> If they aren't synced you should *already* have dirty VCPU_EXREG_PDPTR and pending KVM_REQ_LOAD_MMU_PGD, shouldn't you?
> As long as the caching invariants are respected, this patch is fairly safe, and if they aren't there are plenty of
> preexisting bugs anyway.
>

They can be not synced in other side: not available.

If (!kvm_register_is_available(vcpu, VCPU_EXREG_PDPTR))
it will make no sense to compare mmu->pdptrs when EPT is enabled.

Because vmcs might have different copy, it is better to just mark it
dirty in load_pdptrs().

(SVM is OK even with NPT enabled, since vmcb doesn't have a copy)

I haven't investigated enough then and today. It is quit complicated.

Thanks
Lai

>
>> The next patch does add a fairly heavy unload of the current root for
>> !TDP, but that's a bug fix and should be ordered before any
>> optimizations anyways.