linux-kernel - Re: "KVM: x86/mmu: Overhaul TDP MMU zapping and flushing" breaks SVM on Hyper-V

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Date:   Fri, 24 Feb 2023 17:26:16 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tianyu Lan <ltykernel@...il.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
Subject: Re: "KVM: x86/mmu: Overhaul TDP MMU zapping and flushing" breaks SVM
 on Hyper-V

On 2/24/23 17:17, Jeremi Piotrowski wrote:
>>>>>>> Depending on the performance results of adding the hypercall to
>>>>>>> svm_flush_tlb_current, the fix could indeed be to just disable usage of
>>>>>>> HV_X64_NESTED_ENLIGHTENED_TLB.
>>>>>> Minus making nested SVM (L3) mutually exclusive, I believe this will do the trick:
>>>>>>
>>>>>> +       /* blah blah blah */
>>>>>> +       hv_flush_tlb_current(vcpu);
>>>>>> +
>>>>> Yes, it's either this or disabling the feature.
>>>>>
>>>>> Paolo
>>>> Combining the two sub-threads: both of the suggestions:
>>>>
>>>> a) adding a hyperv_flush_guest_mapping(__pa(root->spt) after kvm_tdp_mmu_get_vcpu_root_hpa's call to tdp_mmu_alloc_sp()
>>>> b) adding a hyperv_flush_guest_mapping(vcpu->arch.mmu->root.hpa) to svm_flush_tlb_current()
>>>>
>>>> appear to work in my test case (L2 vm startup until panic due to missing rootfs).
>>>>
>>>> But in both these cases (and also when I completely disable HV_X64_NESTED_ENLIGHTENED_TLB)
>>>> the runtime of an iteration of the test is noticeably longer compared to tdp_mmu=0.
>>> Hmm, what is test doing?
>> Booting through OVMF and kernel with no rootfs provided, and panic=-1 specified on the
>> kernel command line. It's a pure startup time test.
>>
> Hi Sean,
> 
> Have you been able to reproduce this by any chance?
> 
> I would be glad to see either of the two fixes getting merged (b) or a) if it doesn't require
> special L3 nested handling) in order to get this regression resolved.

For now the easiest course of action is to just disable the TDP MMU if 
HV_X64_NESTED_ENLIGHTENED_TLB is available.

Paolo