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

[Alexander Graf <agraf@suse.de>]

On 28.07.14 17:03, David Hildenbrand wrote:
>> 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?

Not really I think. We could create a new interrupt_request bit called 
CPU_INTERRUPT_STOPPED that doesn't get unset automatically and simply 
sets cpu->halted = 1 (similar to CPU_INTERRUPT_HALT).

>
> 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).

That's perfectly normal behavior. Just make it two different interrupt 
types:

if (interrupt_request & CPU_INTERRUPT_STOPPED) {
   /* Go back to halted state */
   ...
} else if (interrupt_request & CPU_INTERRUPT_SIGP) {
   env->interrupt_request &= ~CPU_INTERRUPT_STOPPED;
   /* swap PSW */
   ...
} else if ((interrupt_request & CPU_INTERRUPT_HARD) &&
(env->psw.mask & PSW_MASK_EXT)) {
   ...
}

>
> 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)...

TCG is single-threaded :). And if you stick to the interrupt logic above 
all should be good.


Alex