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[109.42.112.224]) by smtp.gmail.com with ESMTPSA id k21-20020a7bc315000000b003fe13c3ece7sm3606294wmj.10.2023.08.17.10.09.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 17 Aug 2023 10:09:50 -0700 (PDT) Message-ID: Date: Thu, 17 Aug 2023 19:09:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [PATCH] chardev/char-pty: Avoid losing bytes when the other side just (re-)connected Content-Language: en-US To: =?UTF-8?Q?Marc-Andr=c3=a9_Lureau?= , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= Cc: qemu-devel@nongnu.org, Paolo Bonzini , Marc Hartmayer References: <20230816210743.1319018-1-thuth@redhat.com> <2bb2f8ac-43b4-9505-c163-d29964bf6a30@redhat.com> From: Thomas Huth In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -60 X-Spam_score: -6.1 X-Spam_bar: ------ X-Spam_report: (-6.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.021, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-4.01, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.29 Precedence: list List-Id: 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 17/08/2023 15.47, Marc-André Lureau wrote: > Hi > > On Thu, Aug 17, 2023 at 5:06 PM Daniel P. Berrangé wrote: >> >> On Thu, Aug 17, 2023 at 02:00:26PM +0200, Thomas Huth wrote: >>> On 17/08/2023 12.32, Daniel P. Berrangé wrote: >>>> On Wed, Aug 16, 2023 at 11:07:43PM +0200, Thomas Huth wrote: >>>>> When starting a guest via libvirt with "virsh start --console ...", >>>>> the first second of the console output is missing. This is especially >>>>> annoying on s390x that only has a text console by default and no graphical >>>>> output - if the bios fails to boot here, the information about what went >>>>> wrong is completely lost. >>>>> >>>>> One part of the problem (there is also some things to be done on the >>>>> libvirt side) is that QEMU only checks with a 1 second timer whether >>>>> the other side of the pty is already connected, so the first second of >>>>> the console output is always lost. >>>>> >>>>> This likely used to work better in the past, since the code once checked >>>>> for a re-connection during write, but this has been removed in commit >>>>> f8278c7d74 ("char-pty: remove the check for connection on write") to avoid >>>>> some locking. >>>>> >>>>> To ease the situation here at least a little bit, let's check with g_poll() >>>>> whether we could send out the data anyway, even if the connection has not >>>>> been marked as "connected" yet. The file descriptor is marked as non-blocking >>>>> anyway since commit fac6688a18 ("Do not hang on full PTY"), so this should >>>>> not cause any trouble if the other side is not ready for receiving yet. >>>>> >>>>> With this patch applied, I can now successfully see the bios output of >>>>> a s390x guest when running it with "virsh start --console" (with a patched >>>>> version of virsh that fixes the remaining issues there, too). >>>>> >>>>> Reported-by: Marc Hartmayer >>>>> Signed-off-by: Thomas Huth >>>>> --- >>>>> chardev/char-pty.c | 22 +++++++++++++++++++--- >>>>> 1 file changed, 19 insertions(+), 3 deletions(-) >>>>> >>>>> diff --git a/chardev/char-pty.c b/chardev/char-pty.c >>>>> index 4e5deac18a..fad12dfef3 100644 >>>>> --- a/chardev/char-pty.c >>>>> +++ b/chardev/char-pty.c >>>>> @@ -106,11 +106,27 @@ static void pty_chr_update_read_handler(Chardev *chr) >>>>> static int char_pty_chr_write(Chardev *chr, const uint8_t *buf, int len) >>>>> { >>>>> PtyChardev *s = PTY_CHARDEV(chr); >>>>> + GPollFD pfd; >>>>> + int rc; >>>>> - if (!s->connected) { >>>>> - return len; >>>>> + if (s->connected) { >>>>> + return io_channel_send(s->ioc, buf, len); >>>>> } >>>>> - return io_channel_send(s->ioc, buf, len); >>>>> + >>>>> + /* >>>>> + * The other side might already be re-connected, but the timer might >>>>> + * not have fired yet. So let's check here whether we can write again: >>>>> + */ >>>>> + pfd.fd = QIO_CHANNEL_FILE(s->ioc)->fd; >>>>> + pfd.events = G_IO_OUT; >>>>> + pfd.revents = 0; >>>>> + rc = RETRY_ON_EINTR(g_poll(&pfd, 1, 0)); >>>>> + g_assert(rc >= 0); >>>>> + if (!(pfd.revents & G_IO_HUP) && (pfd.revents & G_IO_OUT)) { >>>> >>>> Should (can?) we call >>>> >>>> pty_chr_state(chr, 1); >>>> >>>> here ? >>> >>> As far as I understood commit f8278c7d74c6 and f7ea2038bea04628, this is not >>> possible anymore since the lock has been removed. >>> >>>>> + io_channel_send(s->ioc, buf, len); >>>> >>>> As it feels a little dirty to be sending data before setting the >>>> 'connected == 1' and thus issuing the 'CHR_EVENT_OPENED' event >>> >>> I didn't find a really better solution so far. We could maybe introduce a >>> buffer in the char-pty code and store the last second of guest output, but >>> IMHO that's way more complex and thus somewhat ugly, too? >> >> The orignal commit f8278c7d74c6 said >> >> [quote] >> char-pty: remove the check for connection on write >> >> This doesn't help much compared to the 1 second poll PTY >> timer. I can't think of a use case where this would help. >> [/quote] >> >> We've now identified a use case where it is actually important. >> >> IOW, there's a justification to revert both f7ea2038bea04628 and >> f8278c7d74c6, re-adding the locking and write update logic. > > Indeed. But isn't it possible to watch for IO_OUT and get rid of the timer? It might be possible - Marc Hartmayer just sent me a draft patch today that uses qio_channel_add_watch() and gets rid of the timer ... I'll do some experiments with that and send it out if it works reliably. Thomas