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Date:   Tue, 20 Apr 2021 17:24:37 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Ashish Kalra <Ashish.Kalra@amd.com>, tglx@linutronix.de,
        mingo@redhat.com, hpa@zytor.com, joro@8bytes.org, bp@suse.de,
        thomas.lendacky@amd.com, x86@kernel.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, srutherford@google.com,
        venu.busireddy@oracle.com, brijesh.singh@amd.com
Subject: Re: [PATCH v13 08/12] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS
 hypercall
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References: <cover.1618498113.git.ashish.kalra@amd.com>
 <93d7f2c2888315adc48905722574d89699edde33.1618498113.git.ashish.kalra@amd.com>
 <6e6b4e8c-bbfa-fd58-c1e8-895a157762fe@redhat.com>
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On Tue, Apr 20, 2021, Paolo Bonzini wrote:
> On 15/04/21 17:57, Ashish Kalra wrote:
> > From: Ashish Kalra <ashish.kalra@amd.com>
> > 
> > This hypercall is used by the SEV guest to notify a change in the page
> > encryption status to the hypervisor. The hypercall should be invoked
> > only when the encryption attribute is changed from encrypted -> decrypted
> > and vice versa. By default all guest pages are considered encrypted.
> > 
> > The hypercall exits to userspace to manage the guest shared regions and
> > integrate with the userspace VMM's migration code.
> 
> I think this should be exposed to userspace as a capability, rather than as
> a CPUID bit.  Userspace then can enable the capability and set the CPUID bit
> if it wants.
> 
> The reason is that userspace could pass KVM_GET_SUPPORTED_CPUID to
> KVM_SET_CPUID2 and the hypercall then would break the guest.

Right, and that's partly why I was advocating that KVM emulate the MSR as a nop.