> > > > On 28.07.2014, at 16:16, David Hildenbrand <d...@linux.vnet.ibm.com> wrote: > > > > >> > > >> On 10.07.14 15:10, Christian Borntraeger wrote: > > >>> From: David Hildenbrand <d...@linux.vnet.ibm.com> > > >>> > > >>> If a cpu is stopped, it must never be allowed to run and no interrupt > > >>> may wake it > > >>> up. A cpu also has to be unhalted if it is halted and has work to do - > > >>> this > > >>> scenario wasn't hit in kvm case yet, as only "disabled wait" is > > >>> processed within > > >>> QEMU. > > >>> > > >>> Signed-off-by: David Hildenbrand <d...@linux.vnet.ibm.com> > > >>> Reviewed-by: Cornelia Huck <cornelia.h...@de.ibm.com> > > >>> Reviewed-by: Christian Borntraeger <borntrae...@de.ibm.com> > > >>> Signed-off-by: Christian Borntraeger <borntrae...@de.ibm.com> > > >> > > >> This looks like it's something that generic infrastructure should take > > >> care of, no? How does this work for the other archs? They always get an > > >> interrupt on the transition between !has_work -> has_work. Why don't we > > >> get one for s390x? > > >> > > >> > > >> Alex > > >> > > >> > > > > > > Well, we have the special case on s390 as a CPU that is in the STOPPED or > > > the > > > CHECK STOP state may never run - even if there is an interrupt. It's > > > basically like this CPU has been switched off. > > > > > > Imagine that it is tried to inject an interrupt into a stopped vcpu. It > > > will kick the stopped vcpu and thus lead to a call to > > > "kvm_arch_process_async_events()". We have to deny that this vcpu will > > > ever > > > run as long as it is stopped. It's like a way to "suppress" the > > > interrupt for such a transition you mentioned. > > > > An interrupt kick usually just means we go back into the main loop. From > > there we check the interrupt bitmap which interrupt to handle. Check out > > the handling code here: > > > > > > http://git.qemu.org/?p=qemu.git;a=blob;f=cpu-exec.c;h=38e5f02a307523d99134f4e2e6c51683bb10b45b;hb=HEAD#l580 > > > > If you just check for the stopped state in here, do_interrupt() will never > > get called and thus the CPU shouldn't ever get executed. Unless I'm heavily > > mistaken :). > > So you would rather move the check out of has_work() into the main loop in > cpu-exec.c and directly into kvm_arch_process_async_events()? > > This would on the other hand lead to an unhalt of the vcpu in cpu_exec() on > any > CPU_INTERRUPT_HARD. A VCPU might thus be unhalted although it is not able to > run. Is okay? > > Looking at cpu.c:cpu_thread_is_idle(), we would maybe return false, although > we > are idle (because we are idle when we are stopped)? > > My qemu kvm knowledge is way better than the qemu emulation knowledge, so I > appreciate any insights :) > > > > > > > > > Later, another vcpu might decide to turn that vcpu back on (by e.g. > > > sending a > > > SIGP START to that vcpu). > > > > Yes, in that case that other CPU generates a signal (a different bit in > > interrupt_request) and the first CPU would see that it has to wake up and > > wake up. > > > > > I am not sure if such a mechanism/scenario is applicable to any other > > > arch. They > > > all seem to reset the cs->halted flag if they know they are able to run > > > (e.g. > > > due to an interrupt) - they have no such thing as "stopped cpus", only > > > "halted/waiting cpus". > > > > There's not really much difference between the two. The only difference > > from a software point of view is that a "stopped" CPU has its external > > interrupt bits masked off, no? > > Well the difference is, that a STOPPED vcpu can be woken up by non-interrupt > like things (SIGP START) AND a special interrupt (SIGP RESTART - which is like > a "SIPI"++ as it performs a psw exchange - "NMI"). So we basically have two > paths that can lead to a state change. All interrupt bits may be in any > combination (SIGP RESTART interrupts can't be masked out, nor can SIGP START > be > denied). > > The other thing may be that on s390, each vcpu (including itself) can put > another vcpu into the STOPPED state - I assume that this is different for x86 > " > INIT_RECEIVED". For this reason we have to watch out for bad race conditions > (e.g. multiple vcpus working on another vcpu)...
Ah, sorry, just to clearify, a vcpu always sets itself to STOPPED, its the other vcpus that trigger it (= interrupt-like). David > > David > > > > > > > Alex > > > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
