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Subject: Re: [PATCH v3 2/2] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS
 hypercall
To:     Sean Christopherson <seanjc@google.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        srutherford@google.com, joro@8bytes.org, brijesh.singh@amd.com,
        thomas.lendacky@amd.com, ashish.kalra@amd.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@suse.de>,
        x86@kernel.org
References: <20210429104707.203055-1-pbonzini@redhat.com>
 <20210429104707.203055-3-pbonzini@redhat.com> <YIxkTZsblAzUzsf7@google.com>
 <c4bf8a05-ec0d-9723-bb64-444fe1f088b5@redhat.com>
 <YJF/3d+VBfJKqXV4@google.com>
 <f7300393-6527-005f-d824-eed5f7f2f8a8@redhat.com>
 <YJGvrYWLQwiRSNLt@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <55db8e64-763b-9ecc-9c9a-6d840628e763@redhat.com>
Date:   Tue, 4 May 2021 22:56:28 +0200
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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