[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [PATCH] hw/char/pl011: Output characters using best-effort mode

From: Marc Zyngier
Subject: Re: [PATCH] hw/char/pl011: Output characters using best-effort mode
Date: Thu, 20 Feb 2020 09:10:11 +0000
User-agent: Roundcube Webmail/1.3.10

On 2020-02-20 06:01, Gavin Shan wrote:
Currently, PL011 is used by ARM virt board by default. It's possible to
block the system from booting. With below parameters in command line, the
backend could run into endless attempts of transmitting packets, which
can't succeed because of running out of sending buffer. The socket might
be not accepted n server side. It's not correct because disconnected
serial port shouldn't stop the system from booting.

   -machine virt,gic-version=3 -cpu max -m 4096
   -monitor none -serial tcp:

The issue can be reproduced by starting a program which listens on TCP
port 50900 and then sleep without accepting any incoming connections. On
the other hand, a VM is started with above parameters and modified qemu
where the PL011 is flooded with 5000K data after it's created. Eventually, the flooding won't proceed and stops after transmitting 2574K data. It's basically to simulate tons of output from EDK-II and demonstrates how the
tons of output can block the system from booting.

This fixes the issue by using newly added API qemu_chr_fe_try_write_all(), which provides another type of service (best-effort). It's different from
qemu_chr_fe_write_all() as the data will be dropped if the backend has
been running into so-called broken state or 50 attempts of transmissions.
The broken state is cleared if the data is transmitted at once.

I don't think dropping the serial port output is an acceptable outcome.

If someone decides to log their console with something that is very slow
(because they decide to carve every bit of it into stone), it shouldn't
be QEMU's decision to just give up on it. Specially if the console is
over TCP, which garantees no loss of data. Someone wanting to have the
behaviour you describe would probably use UDP as the transport protocol
and deal with the consequences.

Similarly, QEMU doesn't drop data on the floor when a write to a disk
image that results in a block allocation fails because the host filesystem
is full.


Jazz is not dead. It just smells funny...

reply via email to

[Prev in Thread] Current Thread [Next in Thread]