linux-kernel - Re: [PATCH 1/4] KVM: x86: Add AMD SEV specific Hypercall3

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Message-ID: <YIIO0wtHeNK6pyri@google.com>
Date:   Fri, 23 Apr 2021 00:03:31 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ashish Kalra <Ashish.Kalra@....com>
Cc:     pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, joro@...tes.org, bp@...e.de,
        thomas.lendacky@....com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, srutherford@...gle.com,
        venu.busireddy@...cle.com, brijesh.singh@....com
Subject: Re: [PATCH 1/4] KVM: x86: Add AMD SEV specific Hypercall3

On Thu, Apr 22, 2021, Ashish Kalra wrote:
> From: Brijesh Singh <brijesh.singh@....com>
> 
> KVM hypercall framework relies on alternative framework to patch the
> VMCALL -> VMMCALL on AMD platform. If a hypercall is made before
> apply_alternative() is called then it defaults to VMCALL. The approach
> works fine on non SEV guest. A VMCALL would causes #UD, and hypervisor
> will be able to decode the instruction and do the right things. But
> when SEV is active, guest memory is encrypted with guest key and
> hypervisor will not be able to decode the instruction bytes.
> 
> Add SEV specific hypercall3, it unconditionally uses VMMCALL. The hypercall
> will be used by the SEV guest to notify encrypted pages to the hypervisor.

I still think we should invert the default and avoid having an SEV specific
variant of kvm_hypercall3().

https://lore.kernel.org/kvm/X8gyhCsEMf8QU9H%2F@google.com/