Re: [Qemu-devel] [mttcg] cputlb: Use async tlb_flush_by_mmuidx

[Paolo Bonzini]

On 29/02/2016 15:02, alvise rigo wrote:
> > Yeah, that's the other approach -- really split the things that can
> > be async and do real "wait for completion" at points which must
> > synchronize. (Needs a little care since DMB is not the only such point.)
> > An initial implementation that does an immediate wait-for-completion
> > is probably simpler to review though, and add the real asynchrony
> > later. And either way you need an API for the target to wait for
> > completion.
> OK, so basically being sure that the target CPU performs the flush
> before executing the next TB is not enough. We need a sort of feedback
> that the flush has been done before emulating the next guest
> instruction. Did I get it right?

That risks getting deadlocks if CPU A asks B to flush the TLB and vice
versa.  Using a halted state means that the VCPU thread goes through the
cpus.c loop and can for example service other CPUs' TLB flush requests.

Paolo