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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: Fri, 21 Feb 2020 09:09:49 +0000
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Hi Gavin,

On 2020-02-21 04:24, Gavin Shan wrote:
Hi Peter and Marc,

On 2/20/20 9:10 PM, Peter Maydell wrote:
On Thu, 20 Feb 2020 at 09:10, Marc Zyngier <address@hidden> wrote:
On 2020-02-20 06:01, Gavin Shan wrote:
This fixes the issue by using newly added API
which provides another type of service (best-effort). It's different
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
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.

Agreed. The correct fix for this is the one cryptically described
in the XXX comment this patch deletes:

-        /* XXX this blocks entire thread. Rewrite to use
-         * qemu_chr_fe_write and background I/O callbacks */

The idea is that essentially we end up emulating the real
hardware's transmit FIFO:
  * as data arrives from the guest we put it in the FIFO
  * we try to send the data with qemu_chr_fe_write(), which does
    not block
  * if qemu_chr_fe_write() tells us it did not send all the data,
    we use qemu_chr_fe_add_watch() to set up an I/O callback
    which will get called when the output chardev has drained
    enough that we can try again
  * we make sure all the guest visible registers and mechanisms
    for tracking tx fifo level (status bits, interrupts, etc) are
    correctly wired up

Then we don't lose data or block QEMU if the guest sends
faster than the chardev backend can handle, assuming the
guest is well-behaved -- just as with a real hardware slow
serial port, the guest will fill the tx fifo and then either poll
or wait for an interrupt telling it that the fifo has drained
before it tries to send more data.

There is an example of this in hw/char/cadence_uart.c
(and an example of how it works for a UART with no tx
fifo in hw/char-cmsdk-apb-uart.c, which is basically the
same except the 'fifo' is just one byte.)

You will also find an awful lot of XXX comments like the
above one in various UART models in hw/char, because
converting an old-style simple blocking UART implementation
to a non-blocking one is a bit fiddly and needs knowledge
of the specifics of the UART behaviour.

The other approach here would be that we could add
options to relevant chardev backends so the user
could say "if you couldn't connect to the tcp server I
specified, throw away data rather than waiting", where
we don't have suitable options already. If the user specifically
tells us they're ok to throw away the serial data, then it's
fine to throw away the serial data :-)

I was intended to convince Marc that it's fine to lose data if the
serial connection is broken with an example. Now, I'm taking the
example trying to convince both of you: Lets assume we have a ARM
board and the UART (RS232) cable is unplugged and plugged in the middle of
system booting. I think we would get some output lost. We're emulating
pl011 and I think it would have same behavior. However, I'm not sure
if it makes sense :)

But the case you describe in the commit message is not that one.
The analogy is that of a serial port *plugged* and asserting flow control.

Another thing is that the "system" as been constructed this way by the
user. QEMU is not in a position to choose and output what is convenient,
when it is convenient. In my world, the serial output is absolutely
crucial. This is where I look for clues about failures and odd behaviours, and I rely on the serial port emulation to be 100% reliable (and for what it's worth, the Linux kernel can output to the serial port asynchronously,
to some extent).


If above analysis is correct and the first approach doesn't work out. We have to consider the 2nd approach - adding option to backend to allow losing data. I'm going to add "allow-data-lost" option for TYPE_CHARDEV_SOCKET. With the option, a back-off algorithm in tcp_chr_write(): The channel is consider as broken if it fails to transmit data in last continuous 5 times. The transmission is still issued when the channel is in broken state and recovered to normal state if
transmission succeeds for once.

That'd be an option if you could configure the UART with something that says "no flow control". In that case, dropping data on the floor becomes perfectly
acceptable, as it requires buy-in from the user.


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

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