Re: [Qemu-devel] [PATCH/RFC 4/5] s390x/kvm: test whether a cpu is STOPPED when checking "has_work"

[David Hildenbrand <dahi@linux.vnet.ibm.com>]

> > 
> > On 28.07.2014, at 16:16, David Hildenbrand <dahi@linux.vnet.ibm.com> wrote:
> > 
> > >> 
> > >> On 10.07.14 15:10, Christian Borntraeger wrote:
> > >>> From: David Hildenbrand <dahi@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 <dahi@linux.vnet.ibm.com>
> > >>> Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> > >>> Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
> > >>> Signed-off-by: Christian Borntraeger <borntraeger@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
> > 
>