* Stefan Hajnoczi (stefa...@gmail.com) wrote: > On Wed, Sep 6, 2017 at 4:14 PM, Dr. David Alan Gilbert > <dgilb...@redhat.com> wrote: > > * Stefan Hajnoczi (stefa...@gmail.com) wrote: > >> On Wed, Aug 23, 2017 at 02:51:03PM +0800, Peter Xu wrote: > >> > The root problem is that, monitor commands are all handled in main > >> > loop thread now, no matter how many monitors we specify. And, if main > >> > loop thread hangs due to some reason, all monitors will be stuck. > >> > >> I see a larger issue with postcopy: existing QEMU code assumes that > >> guest memory access is instantaneous. > >> > >> Postcopy breaks this assumption and introduces blocking points that can > >> now take unbounded time. > >> > >> This problem isn't specific to the monitor. It can also happen to other > >> components in QEMU like the gdbstub. > >> > >> Do we need an asynchronous memory API? Synchronous memory access should > >> only be allowed in vcpu threads. > > > > It would probably be useful for gdbstub where the overhead of async > > doesn't matter; but doing that for all IO emulation is hard. > > Why is it hard? > > Memory access can be synchronous in the vcpu thread. That eliminates > a lot of code straight away. > > Anything using dma-helpers.c is already async. They just don't know > that the memory access part is being made async too :).
Can you point me to some info on that ? > The remaining cases are virtio and some other devices. > > If you are worried about performance, the first rule is that async > memory access is only needed on the destination side when post-copy is > active. Maybe use setjmp to return from the signal handler and queue > a callback for when the page has been loaded. I'm not sure it's worth trying to be too clever at avoiding this; I see the fact that we're doing IO with the bql held as a more fundamental problem. Dave > Stefan -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
