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[109.43.177.123]) by smtp.gmail.com with ESMTPSA id bu24-20020a170906a15800b0098669cc16b2sm10122085ejb.83.2023.09.27.22.18.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 27 Sep 2023 22:18:02 -0700 (PDT) Message-ID: <640e768f-614e-53ee-dcea-3a1844c66c09@redhat.com> Date: Thu, 28 Sep 2023 07:18:00 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH] chardev/char-pty: Avoid losing bytes when the other side just (re-)connected Content-Language: en-US From: Thomas Huth 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 , Amaury Pouly References: <20230816210743.1319018-1-thuth@redhat.com> <2bb2f8ac-43b4-9505-c163-d29964bf6a30@redhat.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-1.473, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=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 28/08/2023 14.23, Thomas Huth wrote: > On 17/08/2023 19.09, Thomas Huth wrote: >> 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. > > I did quite a bunch of experiments with the code, but as far as I can see, > it's not so straight forward: IO_OUT keeps being enabled once IO_HUP occurs > since the kernel can still buffer some output before it de-asserts IO_OUT. > So there is no easy way to check for a re-connection without some kind of > regular polling, as far as I can see... > > Well, maybe something like this might be possible: Add a watch for IO_HUP. > Once HUP occurs, don't signal CHR_EVENT_CLOSED yet, but set a flag variable > and in case the guest still writes something to the chardev and the flag is > true, we write the bytes to the pty one by one (to avoid locking in case the > buffer gets full) and check for IO_OUT via g_poll() after each write. Once > we see IO_OUT being gone, we can signal CHR_EVENT_CLOSED and add a watch for > IO_OUT being enabled again. Once that watch triggers, we know that the other > side re-connected and we can signal CHR_EVENT_OPENED and continue normally. > ... sounds all quite complicated, is it worth the effort? What do you think? Ping! Marc-André, Daniel, any thoughts on this? Would such a rewrite be worth the effort? Or could we just go ahead with my simple patch here? Or would you prefer reverting the locking patches? Thomas