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Revision d185c094

IDd185c094b404b4ff392b77d1244c0233da7d53bd

Added by Paolo Bonzini about 11 years ago

qemu-char: eliminate busy waiting on can_read returning zero

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. :) Without
this patch, emulation will hang.

Signed-off-by: Paolo Bonzini <>
Message-id:
Signed-off-by: Anthony Liguori <>

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