Re: [Qemu-devel] [6391] Fix nographic mode and VNC

[Paul Brook <paul@codesourcery.com>]

> >> A second solution is to use a polling select() in cpu_exec.  Since
> >> you're adding a system call (and a rather heavy one) in the fast path,
> >> this is going to likely hurt TCG performance.
> >
> > This won't work. If the guest really is in a tight loop then TB chaining
> > means it will never exit translated code.
>
> But then signal delivery wouldn't either, right?  That suggests that if
> the guest is in a tight loop right now, QEMU will freeze.

No. The signal handler calls cpu_interrupt, which unlinks the TBs.

> There's a fair bit of code that is safe to run along side of TCG.  If we
> separate the locking for the device model code from every thing (the
> monitor, vnc, sdl, etc.), then we could still let QEMU be responsive
> even in such a condition.

Maybe. You risk having to put an SMP safe lock in the MMIO handler, which 
would probably do bad things to performance. Many of the embedded targets 
don't have DMA capable peripherals, so we want to avoid making MMIO too 
expensive. kvm is a bit different because MMIO is already horribly expensive.

Paul