Hi Paolo, On Sat, Apr 6, 2013 at 1:59 AM, Paolo Bonzini <pbonz...@redhat.com> 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