Re: [Qemu-devel] [PATCH] 8250: more realistic TX-done IRQ rate

[Paul Brook <paul@codesourcery.com>]

On Saturday 12 April 2008, Jan Kiszka wrote:
> The 8250 UART emulation currently raises a TX-done IRQ immediately when the
> guest writes out some character. This is problematic for guests like Linux
> which may flush its output buffer in a loop from IRQ context, because they
> may then enter a tight loop with IRQs disabled. In fact, Linux breaks out
> of this loop after some iterations and issue the well-known [1] "too much
> work for irq..." warning. And in case the console output is on the very
> same serial port, the console output is utterly corrupted.

Please see previous threads on this topic.

> Patch below addresses the issue by delaying the TX-done IRQ more
> realistically, ie. according to the currently set baudrate.

Unless the serial baud rate is extremely low (<1kbaud) this isn't going to 
work with any sort of reliability. You have to fix this in a way that doesn't 
require high resolution realtime response.

I'd also expect you to have the same problem with the RX queue. If you don't 
it's a bug elsewhere in qemu.

> +            qemu_mod_timer(s->tx_timer, 1000 / (11520 / s->divider));

This looks bogus. I think you're a few orders of magnitude out in your timing 
calculations. As mentioned above, in practice you unlikely to get anywhere 
near the necessary realtime performance out of qemu.

Paul