linux-kernel - Re: [PATCH Part2 RFC v4 21/40] KVM: SVM: Add initial SEV-SNP support

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Message-ID: <fb54b509-29a3-f2d2-5a23-eb8d9d661fac@amd.com>
Date:   Fri, 16 Jul 2021 16:03:57 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     brijesh.singh@....com, 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 21/40] KVM: SVM: Add initial SEV-SNP support


On 7/16/21 2:31 PM, Sean Christopherson wrote:
> That's not what I was asking.  My question is if KVM will break/fail if someone
> runs a KVM build with SNP enabled halfway through the series.  E.g. if I make a
> KVM build at patch 22, "KVM: SVM: Add KVM_SNP_INIT command", what will happen if
> I attempt to launch an SNP guest?  Obviously it won't fully succeed, but will KVM
> fail gracefully and do all the proper cleanup?  Repeat the question for all patches
> between this one and the final patch of the series.
>
> SNP simply not working is ok, but if KVM explodes or does weird things without
> "full" SNP support, then at minimum the module param should be off by default
> until it's safe to enable.  E.g. for the TDP MMU, I believe the approach was to
> put all the machinery in place but not actually let userspace flip on the module
> param until the full implementation was ready.  Bisecting and testing the
> individual commits is a bit painful because it requires modifying KVM code, but
> on the plus side unrelated bisects won't stumble into a half-baked state.

There is one to two patches where I can think of that we may break the
KVM if SNP guest is created before applying the full series. In one
patch we add LAUNCH_UPDATE but reclaim is done in next patch. I like
your idea to push the module init  later in the series.


>
> Ya, got that, but again not what I was asking :-)  Why use cpu_feature_enabled()
> instead of boot_cpu_has()?  As a random developer, I would fully expect that
> boot_cpu_has(X86_FEATURE_SEV_SNP) is true iff SNP is fully enabled by the kernel.

I have to check but I think boot_cpu_has(X64_FEATURE_SEV_SNP) will
return true even when the CONFIG_MEM_ENCRYPT is disabled.


>
>> The approach here is similar to SEV/ES. IIRC, it was done mainly to
>> avoid adding dead code when CONFIG_KVM_AMD_SEV is disabled.
> But this is already in an #ifdef, checking sev_es_guest() is pointless.


Ah Good point.