Message-ID: <7e1d3590-2205-401c-c6f5-e4da534d85a7@linux.microsoft.com>
Date:   Thu, 16 Feb 2023 15:40:20 +0100
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Subject: Re: "KVM: x86/mmu: Overhaul TDP MMU zapping and flushing" breaks SVM
 on Hyper-V
Content-Language: en-US
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Tianyu Lan <ltykernel@gmail.com>,
        "Michael Kelley (LINUX)" <mikelley@microsoft.com>
References: <43980946-7bbf-dcef-7e40-af904c456250@linux.microsoft.com>
 <Y+p1j7tYT+16MX6B@google.com>
 <35ff8f48-2677-78ea-b5f3-329c75ce65c9@redhat.com>
 <Y+qLe42h9ZPRINrG@google.com>
 <CABgObfaZQOvt6v0yGz3MR7FBU7DcrTTGmS6M8RWCX0uy6WML1Q@mail.gmail.com>
 <20d189fc-8d20-8083-b448-460cc0420151@linux.microsoft.com>
 <Y+1ZpxAkome9s1Ve@google.com>
From:   Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
In-Reply-To: <Y+1ZpxAkome9s1Ve@google.com>
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On 15/02/2023 23:16, Sean Christopherson wrote:
> On Tue, Feb 14, 2023, Jeremi Piotrowski wrote:
>> On 13/02/2023 20:56, Paolo Bonzini wrote:
>>> On Mon, Feb 13, 2023 at 8:12 PM Sean Christopherson <seanjc@google.com> 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.

> 
>> So in terms of performance the ranking is (fastest to slowest):
>> 1. tdp_mmu=0 + enlightened TLB
>> 2. tdp_mmu=0 + no enlightened TLB
>> 3. tdp_mmu=1 (enlightened TLB makes minimal difference)