linux-kernel - Re: [PATCH v3 2/2] KVM: X86: Introduce KVM_HC_PAGE_ENC

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Date:   Tue, 4 May 2021 22:56:28 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        srutherford@...gle.com, joro@...tes.org, brijesh.singh@....com,
        thomas.lendacky@....com, ashish.kalra@....com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
        x86@...nel.org
Subject: Re: [PATCH v3 2/2] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS
 hypercall

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