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

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Date:   Tue, 4 May 2021 10:06:47 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Steve Rutherford <srutherford@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, Joerg Roedel <joro@...tes.org>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ashish Kalra <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 ML <x86@...nel.org>
Subject: Re: [PATCH v3 2/2] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS
 hypercall

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