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[79.178.2.19]) by smtp.gmail.com with ESMTPSA id x7sm19348569qkx.110.2020.03.11.04.48.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Mar 2020 04:48:31 -0700 (PDT) Date: Wed, 11 Mar 2020 07:48:26 -0400 From: "Michael S. Tsirkin" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Subject: Re: Upstream QEMU guest support policy ? Re: [PATCH v3 0/2] spapr: Use vIOMMU translation for virtio by default Message-ID: <20200311073145-mutt-send-email-mst@kernel.org> References: <20200305043009.611636-1-david@gibson.dropbear.id.au> <20200310114343.GD3234052@redhat.com> <20200311011247.GT660117@umbus.fritz.box> <20200311100127.GC3614859@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200311100127.GC3614859@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 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: pair@us.ibm.com, aik@ozlabs.ru, qemu-devel@nongnu.org, groug@kaod.org, paulus@samba.org, clg@kaod.org, mdroth@us.ibm.com, qemu-ppc@nongnu.org, David Gibson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Mar 11, 2020 at 10:01:27AM +0000, Daniel P. Berrang=E9 wrote: > On Wed, Mar 11, 2020 at 12:12:47PM +1100, David Gibson wrote: > > On Tue, Mar 10, 2020 at 11:43:43AM +0000, Daniel P. Berrang=E9 wrote: > > > On Thu, Mar 05, 2020 at 03:30:07PM +1100, David Gibson wrote: > > > > Upcoming Secure VM support for pSeries machines introduces some > > > > complications for virtio, since the transfer buffers need to be > > > > explicitly shared so that the hypervisor can access them. > > > >=20 > > > > While it's not strictly speaking dependent on it, the fact that vir= tio > > > > devices bypass normal platform IOMMU translation complicates the is= sue > > > > on the guest side. Since there are some significan downsides to > > > > bypassing the vIOMMU anyway, let's just disable that. > > > >=20 > > > > There's already a flag to do this in virtio, just turn it on by > > > > default for forthcoming pseries machine types. > > >=20 > > > Breaking existing guest OS to support a new secure VM feature that > > > may not even be used/wanted doesn't seems like a sensible tradeoff > > > for default out of the box behaviour. > > >=20 > > > IOW, if Secure VM needs this, can we tie the change in virtio and > > > IOMMU defaults to the machine type flag that enables the use of > > > Secure VM. > >=20 > > There is no such flag. > >=20 > > In the POWER secure VM model, the secure mode option isn't something > > that's constructed in when the hypervisor builds the VM. Instead the > > VM is started normally and transitions itself to secure mode by > > talking directly with the ultravisor (it then uses TPM shenannigans to > > safely get the keys to its real storage backend(s)). >=20 > This is pretty suprising to me. The ability to use secure VM mode surely > depends on host hardware features. We would need to be able to block the > use of this, in order to allow VMs to be live migrated to hosts which > lack the feature. Automatically & silently enabling a feature that > has a hardware dependancy is something we aim to avoid, unless the user > has opted in via some flag (such as -cpu host, or a -cpu $NAME, that > implies the feature). That's something I don't know. Is migration supported in this mode? --=20 MST