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[79.181.91.42]) by smtp.gmail.com with ESMTPSA id w9sm31146642vkd.41.2019.07.31.12.06.14 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Wed, 31 Jul 2019 12:06:16 -0700 (PDT) Date: Wed, 31 Jul 2019 15:06:11 -0400 From: "Michael S. Tsirkin" To: Cornelia Huck Message-ID: <20190731150448-mutt-send-email-mst@kernel.org> References: <20190729125755.45008-1-slp@redhat.com> <20190730133546.056f8b19.cohuck@redhat.com> <09e5ceb5e7c03f74f05307a3b9f9a4df035ff74f.camel@redhat.com> <20190730151400.20686a5b.cohuck@redhat.com> <20190730160605-mutt-send-email-mst@kernel.org> <20190731155551.4bb57ec3.cohuck@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190731155551.4bb57ec3.cohuck@redhat.com> X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.85.217.49 Subject: Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1) 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: peter.maydell@linaro.org, Andrea Bolognani , Sergio Lopez , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Jul 31, 2019 at 03:55:51PM +0200, Cornelia Huck wrote: > On Tue, 30 Jul 2019 16:18:52 -0400 > "Michael S. Tsirkin" wrote: > > > On Tue, Jul 30, 2019 at 03:14:00PM +0200, Cornelia Huck wrote: > > > On Tue, 30 Jul 2019 14:17:48 +0200 > > > Andrea Bolognani wrote: > > > > > > > On Tue, 2019-07-30 at 13:35 +0200, Cornelia Huck wrote: > > > > > On Tue, 30 Jul 2019 12:25:30 +0200 > > > > > Andrea Bolognani wrote: > > > > > > Can you please make sure virtio-mmio uses the existing interface > > > > > > instead of introducing a new one? > > > > > > > > > > FWIW, I really hate virtio-pci's disable-modern/disable-legacy... for a > > > > > starter, what is 'modern'? Will we have 'ultra-modern' in the future? > > > > > > > > AIUI the modern/legacy terminology is part of the VirtIO spec, so > > > > while I agree that it's not necessarily the least prone to ambiguity > > > > at least it's well defined. > > > > > > Legacy is, modern isn't :) Devices/drivers are conforming to the > > > standard, I don't think there's a special term for that. > > > > Right, if we followed the spec, disable-modern would have been > > force-legacy. > > > > I'm fine with adding force-legacy for everyone and asking tools to > > transition if there. Document it's same as disable-modern for pci. > > Cornelia? > > 'force-legacy' is certainly better than 'disable-modern'. Not sure if > it's much of a gain at this point in time, and it does not really add > anything over limiting the revision to 0 for ccw, but I don't really > object to it. > > > > > > > > > > > > > > It is also quite backwards with the 'disable' terminology. > > > > > > > > That's also true. I never claimed the way virtio-pci does it is > > > > perfect! > > > > > > > > > We also have a different mechanism for virtio-ccw ('max_revision', > > > > > which covers a bit more than virtio-1; it doesn't have a 'min_revision', > > > > > as negotiating the revision down is fine), so I don't see why > > > > > virtio-mmio should replicate the virtio-pci mechanism. > > > > > > > > > > Also, IIUC, virtio-mmio does not have transitional devices, but either > > > > > version 1 (legacy) or version 2 (virtio-1). It probably makes more > > > > > sense to expose the device version instead; either as an exact version > > > > > (especially if it isn't supposed to go up without incompatible > > > > > changes), or with some min/max concept (where version 1 would stand a > > > > > bit alone, so that would probably be a bit awkward.) > > > > > > > > I think that if reinventing the wheel is generally agreed not to be > > > > a good idea, then it stands to reason that reinventing it twice can > > > > only be described as absolute madness :) > > > > > > > > We should have a single way to control the VirtIO protocol version > > > > that works for all VirtIO devices, regardless of transport. We might > > > > even want to have virtio-*-{device,ccw}-non-transitional to mirror > > > > the existing virtio-*-pci-non-transitional. > > > > > > > > FWIW, libvirt already implements support for (non)-transitional > > > > virtio-pci devices using either the dedicated devices or the base > > > > virtio-pci plus the disable-{modern,legacy} attributes. > > > > > > One problem (besides my dislike of the existing virtio-pci > > > interfaces :) is that pci, ccw, and mmio all have slightly different > > > semantics. > > > > > > - pci: If we need to keep legacy support around, we cannot enable some > > > features (IIRC, pci-e, maybe others as well.) That means transitional > > > devices are in some ways inferior to virtio-1 only devices, so it > > > makes a lot of sense to be able to configure devices without legacy > > > support. The differences between legacy and virtio-1 are quite large. > > > - ccw: Has revisions negotiated between device and driver; virtio-1 > > > requires revision 1 or higher. (Legacy drivers that don't know the > > > concept of revisions automatically get revision 0.) Differences > > > between legacy and virtio-1 are mostly virtqueue endianness and some > > > control structures. > > > - mmio: Has device versions offered by the device, the driver can take > > > it or leave it. No transitional devices. Differences don't look as > > > large as the ones for pci, either. > > > > > > So, if we were to duplicate the same scheme as for pci for ccw and mmio > > > as well, we'd get > > > > > > - ccw: devices that support revision 0 only (disable-modern), that act > > > as today, or that support at least revision 1 (disable-legacy). We > > > still need to keep max_revision around for backwards compatibility. > > > Legacy only makes sense for compat machines (although this is > > > equivalent to max_revision 0); I don't see a reason why you would > > > want virtio-1 only devices, unless you'd want to rip out legacy > > > support in QEMU completely. > > > > Reduce security attack surface slightly. Save some cycles > > (down the road) on branches in the endian-ness handling. > > Most of that stuff is actually in the core code, right? Ripping out > legacy will have much more impact outside of ccw, I guess. > > > Make sure your guests > > are all up to date in preparation to the day when legacy will go away. > > If legacy goes away, legacy guests will be busted anyway :) It'll take a while for it to go away. But we can try to push guests in the direction of coding up modern support e.g. by forcing modern by default. > (There should not be many, if any, of these -- ccw switched on virtio-1 > by default quite some time ago, and the s390x legacy virtio transport > was s390-virtio anyway :) > > > > > Not a huge win, for sure, but hey - it's something. > > > > > - mmio: devices that support version 1 (disable-modern), or version 2 > > > (disable-legacy). You cannot have both at the same time. Whether this > > > makes sense depends on whether there will be a version 3 in the > > > future. > > > > > > So, this might make some sense for mmio; for ccw, I don't see any > > > advantages other than confusing people further...