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[83.52.55.21]) by smtp.gmail.com with ESMTPSA id b188sm469839wmd.39.2021.09.14.07.58.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Sep 2021 07:58:14 -0700 (PDT) Subject: Re: [PATCH] virtio-balloon: Fix page-poison subsection name To: =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= , "Dr. David Alan Gilbert" , Gerd Hoffmann , Peter Maydell References: <20210914131716.102851-1-dgilbert@redhat.com> <535891c6-237b-6d37-7492-ef8c1e19e6ca@redhat.com> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: <0974971c-2443-8a7d-b5c6-22d2db107b05@redhat.com> Date: Tue, 14 Sep 2021 16:58:13 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=philmd@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=philmd@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -51 X-Spam_score: -5.2 X-Spam_bar: ----- X-Spam_report: (-5.2 / 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, NICE_REPLY_A=-1.969, 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: David Hildenbrand , stefanha@redhat.com, qemu-devel@nongnu.org, Alexander Duyck , mst@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 9/14/21 3:58 PM, Daniel P. Berrangé 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" > > ? IIUC so far only user-creatable devices are required to have a stable name in the TYPE definition (because CLI must stay stable). Which is why this type is not recommended for migration section names.