Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH v3 2/2] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS
 hypercall
To:     Steve Rutherford <srutherford@google.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>, Joerg Roedel <joro@8bytes.org>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Ashish Kalra <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 ML <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>
 <CABayD+f41GQwCL1818S7iogNHO+MLesLJ-hCX5Bbf_0vFfDMrw@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <7c8f388c-1c0e-1fa8-24a5-46ffd40f8b24@redhat.com>
Date:   Tue, 4 May 2021 10:06:47 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.8.1
MIME-Version: 1.0
In-Reply-To: <CABayD+f41GQwCL1818S7iogNHO+MLesLJ-hCX5Bbf_0vFfDMrw@mail.gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 04/05/21 01:22, Steve Rutherford wrote:
> As far as I know, because of MSR filtering, the only "code" that needs
> to be in KVM for MSR handling is a #define reserving the PV feature
> number and a #define for the MSR number.
> 
> Arguably, you don't even need to add the new PV bits to the supported
> cpuid, since MSR filtering is really what determines if kernel support
> is present.

Not only I don't need to do that, I must not. :)

>> At this point I very much prefer the latter, which is basically Ashish's
>> earlier patch.
>
> The minor distinction would be that if you expose the cpuid bit to the
> guest you plan on intercepting the MSR with filters, and would not
> need any handler code in the kernel.

Yep, and it's not a minor distinction after all (especially from the PoV 
of the guy who actually ends up maintaining the code, i.e. me), so 
that's what I'm going for.

Paolo