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[82.29.237.198]) by smtp.gmail.com with ESMTPSA id f25sm479560wml.38.2021.09.07.04.52.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Sep 2021 04:52:56 -0700 (PDT) Date: Tue, 7 Sep 2021 12:52:54 +0100 From: "Dr. David Alan Gilbert" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Subject: Re: [RFC PATCH v2 12/12] i386/sev: update query-sev QAPI format to handle SEV-SNP Message-ID: References: <20210826222627.3556-1-michael.roth@amd.com> <20210826222627.3556-13-michael.roth@amd.com> <87tuj4qt71.fsf@dusky.pond.sub.org> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.7 (2021-05-04) Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.391, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Tom Lendacky , Brijesh Singh , Eduardo Habkost , kvm@vger.kernel.org, "Michael S . Tsirkin" , Connor Kuehl , Michael Roth , James Bottomley , Markus Armbruster , Eric Blake , qemu-devel@nongnu.org, Dov Murik , Paolo Bonzini , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , David Gibson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Daniel P. Berrangé (berrange@redhat.com) wrote: > On Wed, Sep 01, 2021 at 04:14:10PM +0200, Markus Armbruster wrote: > > Michael Roth writes: > > > > > Most of the current 'query-sev' command is relevant to both legacy > > > SEV/SEV-ES guests and SEV-SNP guests, with 2 exceptions: > > > > > > - 'policy' is a 64-bit field for SEV-SNP, not 32-bit, and > > > the meaning of the bit positions has changed > > > - 'handle' is not relevant to SEV-SNP > > > > > > To address this, this patch adds a new 'sev-type' field that can be > > > used as a discriminator to select between SEV and SEV-SNP-specific > > > fields/formats without breaking compatibility for existing management > > > tools (so long as management tools that add support for launching > > > SEV-SNP guest update their handling of query-sev appropriately). > > > > Technically a compatibility break: query-sev can now return an object > > that whose member @policy has different meaning, and also lacks @handle. > > > > Matrix: > > > > Old mgmt app New mgmt app > > Old QEMU, SEV/SEV-ES good good(1) > > New QEMU, SEV/SEV-ES good(2) good > > New QEMU, SEV-SNP bad(3) good > > > > Notes: > > > > (1) As long as the management application can cope with absent member > > @sev-type. > > > > (2) As long as the management application ignores unknown member > > @sev-type. > > > > (3) Management application may choke on missing member @handle, or > > worse, misinterpret member @policy. Can only happen when something > > other than the management application created the SEV-SNP guest (or the > > user somehow made the management application create one even though it > > doesn't know how, say with CLI option passthrough, but that's always > > fragile, and I wouldn't worry about it here). > > > > I think (1) and (2) are reasonable. (3) is an issue for management > > applications that support attaching to existing guests. Thoughts? > > IIUC you can only reach scenario (3) if you have created a guest > using '-object sev-snp-guest', which is a new feature introduced > in patch 2. > > IOW, scenario (3) old mgmt app + new QEMU + sev-snp guest does > not exist as a combination. Thus the (bad) field is actually (n/a) > > So I believe this proposed change is acceptable in all scenarios > with existing deployed usage, as well as all newly introduced > scenarios. I wonder if it's worth going firther and renaming 'policy' in the SNP world to 'snppolicy' just to reduce the risk of accidentally specifying the wrong one. Dave > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK