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[80.230.35.22]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4356996e2d8sm29384021f8f.28.2026.01.20.04.56.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Jan 2026 04:56:34 -0800 (PST) Date: Tue, 20 Jan 2026 07:56:31 -0500 From: "Michael S. Tsirkin" To: Filip Hejsek Cc: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , qemu-devel@nongnu.org, =?iso-8859-1?Q?Marc-Andr=E9?= Lureau , Paolo Bonzini , Laurent Vivier , Amit Shah , Markus Armbruster , Eric Blake , Eduardo Habkost , Marcel Apfelbaum , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Yanan Wang , Zhao Liu , Maximilian Immanuel Brandtner , Szymon Lukasz Subject: Re: [PATCH v6 08/12] virtio-serial-bus: add terminal resize messages Message-ID: <20260120075558-mutt-send-email-mst@kernel.org> References: <20260119-console-resize-v6-0-33a7b0330a7a@gmail.com> <20260119-console-resize-v6-8-33a7b0330a7a@gmail.com> <20260119044148-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.087, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-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: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Jan 20, 2026 at 10:16:08AM +0100, Filip Hejsek wrote: > On Tue, 2026-01-20 at 08:43 +0000, Daniel P. Berrangé wrote: > > On Mon, Jan 19, 2026 at 04:43:21AM -0500, Michael S. Tsirkin wrote: > > > On Mon, Jan 19, 2026 at 09:38:41AM +0000, Daniel P. Berrangé wrote: > > > > On Mon, Jan 19, 2026 at 04:27:51AM +0100, Filip Hejsek wrote: > > > > > Implement the part of the virtio spec that allows to notify the virtio > > > > > driver about terminal resizes. The virtio spec contains two methods to > > > > > achieve that: > > > > > > > > > > For legacy drivers, we have only one port and we put the terminal size > > > > > in the config space and inject the config changed interrupt. > > > > > > > > > > For multiport devices, we use the control virtqueue to send a packet > > > > > containing the terminal size. Note that old versions of the Linux kernel > > > > > used an incorrect order for the fields (rows then cols instead of cols > > > > > then rows), until it was fixed by commit 5326ab737a47278dbd16ed3ee7380b26c7056ddd. > > > > > > > > > > As a result, when using a Linux kernel older than 6.15, the number of rows > > > > > and columns will be swapped. > > > > > > > > > > Based on a patch originally written by Szymon Lukasz , > > > > > but partially rewritten to fix various corner cases. > > > > > > > > > > Signed-off-by: Szymon Lukasz > > > > > Signed-off-by: Filip Hejsek > > > > > --- > > > > > hw/char/trace-events | 1 + > > > > > hw/char/virtio-serial-bus.c | 76 +++++++++++++++++++++++++++++++++++++-- > > > > > hw/core/machine.c | 4 ++- > > > > > include/hw/virtio/virtio-serial.h | 5 +++ > > > > > 4 files changed, 83 insertions(+), 3 deletions(-) > > > > > > > > > > > > > > @@ -1158,6 +1228,8 @@ static const Property virtio_serial_properties[] = { > > > > > 31), > > > > > DEFINE_PROP_BIT64("emergency-write", VirtIOSerial, host_features, > > > > > VIRTIO_CONSOLE_F_EMERG_WRITE, true), > > > > > + DEFINE_PROP_BIT64("console-size", VirtIOSerial, host_features, > > > > > + VIRTIO_CONSOLE_F_SIZE, true), > > > > > }; > > > > > > > > Given the horrible mess with the kernel intentionally changing its > > > > behaviour after 15 years, I don't think we can we set this to be > > > > enabled by default. > > > > > > > > The recent behaviour change is never going to be backported to enough > > > > stable distros that we can rely on the new behaviour, and thanks to > > > > the change we can't rely on the old behaviour either. We're doomed no > > > > matter what ordernig we use. > > > > > > > > Thus, IMHO, this has to stay set to false indefinitely. > > > > > > Not sure. But what we can do is add another flag to detect new kernels. > > > I'll try to think of a good name but suggestions are welcome. > > > > How can we detect the kernel ? There's no feature flag that can be > > negotiated or detected to report the changed kernel behaviour > > AFAICS. We have no visibility of kernel version, and even if we did, > > the possibility of backports would make that unreliable too. The > > inability to auto-detect anything is what makes the kernel behaviour > > change so awful. > > > > We can add a nother qemu flag "console-size-inverted" to flip QEMU > > between the 2 behaviours, but that still won't let us be able to > > enable 'console-size' by default without guaranteed regressions. > > The 'console-size-inverted' flag would merely flip the breakage > > between different groups of guest OS. > > We could add a new virtio feature flag, and by default only enable > resizing when the guest supports this new flag. Kernels that support > the flag would work by default, and kernels that have the correct order > but don't yet support the flag would require manually enabling the > feature. Right. For now. In X years when we see downstreams backporting the feature, things can change. > In any case, we will probably need to add some documentation for the > property. I'm trying to find a good place for it, but it seems that the > virtconsole and virtio-serial-bus devices are not actually documented > anywhere at all. They should probably be documented in the man page, > right? > > > > > > > > static void virtio_serial_class_init(ObjectClass *klass, const void *data) > > > > > diff --git a/hw/core/machine.c b/hw/core/machine.c > > > > > index 6411e68856..50554b8900 100644 > > > > > --- a/hw/core/machine.c > > > > > +++ b/hw/core/machine.c > > > > > @@ -38,7 +38,9 @@ > > > > > #include "hw/acpi/generic_event_device.h" > > > > > #include "qemu/audio.h" > > > > > > > > > > -GlobalProperty hw_compat_10_2[] = {}; > > > > > +GlobalProperty hw_compat_10_2[] = { > > > > > + { "virtio-serial-device", "console-size", "off" }, > > > > > +}; > > > > > const size_t hw_compat_10_2_len = G_N_ELEMENTS(hw_compat_10_2); > > > > > > > > > > GlobalProperty hw_compat_10_1[] = { > > > > With regards, > > Daniel