Re: [Qemu-devel] [PATCH v2] qemu-char: eliminate busy waiting on can_read returning zero

[Peter Crosthwaite]

Hi Paolo,

On Sat, Apr 6, 2013 at 1:59 AM, Paolo Bonzini <address@hidden> wrote:
> The character backend refactoring introduced an undesirable busy wait.
> The busy wait happens if can_read returns zero and there is data available
> on the character device's file descriptor.  Then, the I/O watch will
> fire continuously and, with TCG, the CPU thread will never run.
>
>     1) Char backend asks front end if it can write
>     2) Front end says no
>     3) poll() finds the char backend's descriptor is available
>     4) Goto (1)
>
> What we really want is this (note that step 3 avoids the busy wait):
>
>     1) Char backend asks front end if it can write
>     2) Front end says no
>     3) poll() goes on without char backend's descriptor
>     4) Goto (1) until qemu_chr_accept_input() called
>
>     5) Char backend asks front end if it can write
>     6) Front end says yes
>     7) poll() finds the char backend's descriptor is available
>     8) Backend handler called
>
> After this patch, the IOWatchPoll source and the watch source are
> separated.  The IOWatchPoll is simply a hook that runs during the prepare
> phase on each main loop iteration.  The hook adds/removes the actual
> source depending on the return value from can_read.
>
> A simple reproducer is
>
>     qemu-system-i386 -serial mon:stdio
>
> ... followed by banging on the terminal as much as you can. :)

Mouse wheel paste does the trick nicely for me :)

> Without this patch, emulation will hang.
>

Emulation still hangs for me with this patch. It also plays foul with
Anthonys rework of my patch. The only way I can get it to work is
either mine or Anthonys patch and a revert of this one. I think your
patch adversely affects the timeout logic that Anthonys patch relies
on.

Ping me if you want my replication instructions. I will have to send
images as Microblaze doesn't like a null guest.

Regards,
Peter