A small update on this. I have a working implementation of the "halted state" mechanism for waiting all the pending flushes to be completed. However, the way I'm going back to the cpus.c loop (the while(1) in qemu_tcg_cpu_thread_fn) is a bit convoluted. In the case of the TLB ops that always end the TB, a simple cpu_exit() allows me to go back to the main loop. I think in this case we can also use the cpu_loop_exit(), though making the code a bit more complicated since the PC would require some adjustments.
I wanted then to apply the same "halted state" to the LoadLink helper, since also this one might cause some flush requests. In this case, we can not just call cpu_loop_exit() in that the guest code would miss the returned value. Forcing the LDREX instruction to also end the TB through an empty 'is_jmp' condition did the trick allowing once again to use cpu_exit(). Is there another better solution? Thank you, alvise On Mon, Feb 29, 2016 at 3:18 PM, alvise rigo <a.r...@virtualopensystems.com> wrote: > I see the risk. I will come back with something and let you know. > > Thank you, > alvise > > On Mon, Feb 29, 2016 at 3:06 PM, Paolo Bonzini <pbonz...@redhat.com> > wrote: > > > > > > 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 >
