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[82.29.237.198]) by smtp.gmail.com with ESMTPSA id l15sm1154997wms.38.2021.09.14.07.16.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Sep 2021 07:16:03 -0700 (PDT) Date: Tue, 14 Sep 2021 15:16:01 +0100 From: "Dr. David Alan Gilbert" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Subject: Re: [PATCH] virtio-balloon: Fix page-poison subsection name Message-ID: References: <20210914131716.102851-1-dgilbert@redhat.com> <535891c6-237b-6d37-7492-ef8c1e19e6ca@redhat.com> 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=-0.398, 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: Alexander Duyck , mst@redhat.com, qemu-devel@nongnu.org, stefanha@redhat.com, David Hildenbrand Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Daniel P. Berrangé (berrange@redhat.com) wrote: > On Tue, Sep 14, 2021 at 02:30:06PM +0100, Dr. David Alan Gilbert wrote: > > * David Hildenbrand (david@redhat.com) wrote: > > > On 14.09.21 15:17, Dr. David Alan Gilbert (git) wrote: > > > > From: "Dr. David Alan Gilbert" > > > > > > > > The subsection name for page-poison was typo'd as: > > > > > > > > vitio-balloon-device/page-poison > > > > > > > > Note the missing 'r' in virtio. > > > > > > > > When we have a machine type that enables page poison, and the guest > > > > enables it (which needs a new kernel), things fail rather unpredictably. > > > > > > > > The fallout from this is that most of the other subsections fail to > > > > load, including things like the feature bits in the device, one > > > > possible fallout is that the physical addresses of the queues > > > > then get aligned differently and we fail with an error about > > > > last_avail_idx being wrong. > > > > It's not obvious to me why this doesn't produce a more obvious failure, > > > > but virtio's vmstate loading is a bit open-coded. > > > > > > > > Fixes: 7483cbbaf82 ("virtio-balloon: Implement support for page poison reporting feature") > > > > bz: https://bugzilla.redhat.com/show_bug.cgi?id=1984401 > > > > Signed-off-by: Dr. David Alan Gilbert > > > > --- > > > > hw/virtio/virtio-balloon.c | 2 +- > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c > > > > index 5a69dce35d..c6962fcbfe 100644 > > > > --- a/hw/virtio/virtio-balloon.c > > > > +++ b/hw/virtio/virtio-balloon.c > > > > @@ -852,7 +852,7 @@ static const VMStateDescription vmstate_virtio_balloon_free_page_hint = { > > > > }; > > > > static const VMStateDescription vmstate_virtio_balloon_page_poison = { > > > > - .name = "vitio-balloon-device/page-poison", > > > > + .name = "virtio-balloon-device/page-poison", > > > > .version_id = 1, > > > > .minimum_version_id = 1, > > > > .needed = virtio_balloon_page_poison_support, > > > > > > > > > > Oh, that's very subtle. I wasn't even aware that the prefix really has to > > > match the actual device ... I thought the whole idea of the prefix here was > > > just to make the string unique ... > > > > Subsection naming is *very* critical; the logic is something like: > > 'we're loading the X device' > > a subsection arrives for 'N/P' > > if 'X==N' then it looks in X for subsection P. > > If 'X!=N' then it assumes we've finished loading X > > and P is really for an outer device that X is part of. > > This is horrible. > > Is there value in making this more explicit via a code convention > for .name field initializers. eg instead of > > .name = "virtio-balloon-device/page-poison", > > Prefer > > .name = TYPE_VIRTIO_BALLOON "/page-poison" > > ? I think it might be better to see if the vmstate code can check it and error during saving; certainly this case feels detectable, but I need to keep an eye open for all the other weird cases. 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