linux-kernel - Re: [PATCH Part2 RFC v4 22/40] KVM: SVM: Add KVM_SNP

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Date:   Mon, 19 Jul 2021 20:24:45 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 22/40] KVM: SVM: Add KVM_SNP_INIT command

On Fri, Jul 16, 2021, Brijesh Singh wrote:
> 
> On 7/16/21 2:33 PM, Sean Christopherson wrote:
> > On Wed, Jul 07, 2021, Brijesh Singh wrote:
> >> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> >> index 3fd9a7e9d90c..989a64aa1ae5 100644
> >> --- a/include/uapi/linux/kvm.h
> >> +++ b/include/uapi/linux/kvm.h
> >> @@ -1678,6 +1678,9 @@ enum sev_cmd_id {
> >>  	/* Guest Migration Extension */
> >>  	KVM_SEV_SEND_CANCEL,
> >>  
> >> +	/* SNP specific commands */
> >> +	KVM_SEV_SNP_INIT = 256,
> > Is there any meaning behind '256'?  If not, why skip a big chunk?  I wouldn't be
> > concerned if it weren't for KVM_SEV_NR_MAX, whose existence arguably implies that
> > 0-KVM_SEV_NR_MAX-1 are all valid SEV commands.
> 
> In previous patches, Peter highlighted that we should keep some gap
> between the SEV/ES and SNP to leave room for legacy SEV/ES expansion. I
> was not sure how many we need to reserve without knowing what will come
> in the future; especially recently some of the command additional  are
> not linked to the firmware. I am okay to reduce the gap or remove the
> gap all together.

Unless the numbers themselves have meaning, which I don't think they do, I vote
to keep the arbitrary numbers contiguous.  KVM_SEV_NR_MAX makes me nervous, and
there are already cases of related commands being discontiguous, e.g. KVM_SEND_CANCEL.

Peter or Paolo, any thoughts?