From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42745) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZFMXv-00055V-Mr for qemu-devel@nongnu.org; Wed, 15 Jul 2015 09:16:24 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZFMXu-0007L7-AQ for qemu-devel@nongnu.org; Wed, 15 Jul 2015 09:16:23 -0400 Date: Wed, 15 Jul 2015 16:16:07 +0300 From: "Michael S. Tsirkin" Message-ID: <20150715160821-mutt-send-email-mst@redhat.com> References: <20150713151705-mutt-send-email-mst@redhat.com> <20150713143024.3b102b9c.cornelia.huck@de.ibm.com> <20150713153319-mutt-send-email-mst@redhat.com> <20150713152059.69b373ee.cornelia.huck@de.ibm.com> <20150713182901-mutt-send-email-mst@redhat.com> <20150714194344.529a6ea4.cornelia.huck@de.ibm.com> <20150715135237-mutt-send-email-mst@redhat.com> <20150715134638.065bbd03.cornelia.huck@de.ibm.com> <20150715145257-mutt-send-email-mst@redhat.com> <20150715144351.39348922.cornelia.huck@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150715144351.39348922.cornelia.huck@de.ibm.com> Subject: Re: [Qemu-devel] [PATCH 2/5] virtio-blk: disable scsi passthrough for 1.0 device List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Cornelia Huck Cc: Kevin Wolf , qemu-block@nongnu.org, Jason Wang , qemu-devel@nongnu.org, Stefan Hajnoczi , pbonzini@redhat.com On Wed, Jul 15, 2015 at 02:43:51PM +0200, Cornelia Huck wrote: > On Wed, 15 Jul 2015 15:01:01 +0300 > "Michael S. Tsirkin" wrote: > > > On Wed, Jul 15, 2015 at 01:46:38PM +0200, Cornelia Huck wrote: > > > On Wed, 15 Jul 2015 13:59:00 +0300 > > > "Michael S. Tsirkin" wrote: > > > > > > > On Tue, Jul 14, 2015 at 07:43:44PM +0200, Cornelia Huck wrote: > > > > > > Yes, and that's because as written, transitional devices must set > > > > > > ANY_LAYOUT, but that's incompatible with scsi. > > > > > > > > > > Hm, I had a patch before that dynamically allowed different feature > > > > > sets for legacy or modern, not only a subset. Probably won't apply > > > > > anymore, but I'd like to able to do the following: > > > > > > > > > > - driver reads features without negotiating a revision: driver is > > > > > legacy, offer legacy bits > > > > > - driver negotiates revision 0: dito > > > > > - driver negotiates revision >= 1: driver is modern, offer modern bits > > > > > > > > > > That way we could offer SCSI and !ANY_LAYOUT (if scsi is enabled) in the > > > > > first two cases, and a new qemu could still offer scsi to old guests. > > > > > > > > > > Would it be worth pursuing that idea? > > > > > > > > Frankly, I don't think so: I don't see why it makes sense > > > > to expose more features on the legacy interface than > > > > on the modern one. Imagine updating drivers to fix a bug > > > > and losing some features. How does this make sense? > > > > > > I don't think one should be a strict subset of the other. But I think > > > we don't want to withdraw features from legacy guests on qemu updates > > > either? > > > > Absolutely. For now one has to enable the modern interface > > explicitly. Around 2.5 we might switch that around, we'll > > need to think hard about compatibility at that point. > > In any case, we must definitely keep the old capability for old machine > > types. > > ccw only offers revision 0 (legacy) in 2.4. I plan to introduce > revision 1 in 2.5 and force revision to 0 for 2.4 compatibility (as 2.4 > is the first versioned ccw machine). I was talking about pci here actually. > > > > > > > > > > I think the virtio TC's assumption was that the scsi passthrough was a > > > > bad idea, so in QEMU we only keep it around for legacy devices to avoid > > > > regressions. > > > > > > I'm not opposing this :) > > > > > > > > > > > If you disagree and think transitional devices need the SCSI feature, > > > > either try to convince pbonzini or rewrite the spec youself > > > > to support it in the virtio 1 mode. > > > > > > This seems to boil down to the different meaning of "transitional" for > > > ccw and pci, see the other thread. > > > > Before the revision is negotiated, ccw won't know whether > > it's a legacy driver - is that the difference? > > I'd say it doesn't know whether the driver intends to use the modern > interface. That's also the case for pci. > > Fine, but revision is negotiated way before features are > > probed so why does it make a practical difference? > > Legacy drivers (that don't know about the set-revision command) will > read features without revision negotiation - we need to offer them the > legacy feature set. Right. So simply do if (revision < 1) return features & 0xffffffff and that will do this, will it not? -- MST