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SubjectRe: [PATCH v3 2/2] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall
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On 04/05/21 22:33, Sean Christopherson wrote:
> On Tue, May 04, 2021, Paolo Bonzini wrote:
>> On 04/05/21 19:09, Sean Christopherson wrote:
>>> On Sat, May 01, 2021, Paolo Bonzini wrote:
>>>> - make it completely independent from migration, i.e. it's just a facet of
>>>> MSR_KVM_PAGE_ENC_STATUS saying whether the bitmap is up-to-date. It would
>>>> use CPUID bit as the encryption status bitmap and have no code at all in KVM
>>>> (userspace needs to set up the filter and implement everything).
>>>
>>> If the bit is purely a "page encryption status is up-to-date", what about
>>> overloading KVM_HC_PAGE_ENC_STATUS to handle that status update as well? That
>>> would eliminate my biggest complaint about having what is effectively a single
>>> paravirt feature split into two separate, but intertwined chunks of ABI.
>>
>> It's true that they are intertwined, but I dislike not having a way to read
>> the current state.
>
> From the guest?

Yes, host userspace obviously doesn't need one since it's implemented
through an MSR filter. It may not be really necessary to read it, but
it's a bit jarring compared to how the rest of the PV APIs uses MSRs.

Also from a debugging/crashdump point of view the VMM may have an
established way to read an MSR from a vCPU, but it won't work if you
come up with a new way to set the state.

Paolo

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