Re: [Qemu-devel] [PATCH v1] cpus: track calls to resume/pause_all_vcpus()

[David Hildenbrand <david@redhat.com>]

On 09.04.2018 15:12, Paolo Bonzini wrote:
> On 09/04/2018 15:07, David Hildenbrand wrote:
>> If we have parallel calls to resume/pause_all_vcpus() we can get
>> into trouble because the qemu mutex is temporarily dropped while
>> waiting for all threads to stop. This can happen e.g. for s390x, where
>> resume/pause_all_vcpus() can be triggered by a VCPU.
> 

I'm also using it resume/pause_all_vcpus() now in a prototype to
temporarily get all VCPUs out of KVM, that's how I noticed that this is
shaky :)

> Why does s390 need to do pause_all_vcpus()/resume_all_vcpus() instead of
> just asking the main thread to do it (similar to qemu_system_reset), is
> it because diag 308 must be synchronous?

Christian implemented it back than to (quoting from another mail)

"I did this to prevent a "still running CPU to restart an already
stopped one"."

The problem is that another VCPU could just be about to send a SIGP
START/RESTART to a VCPU. Without the pause_all_vcpus(), the SIGP could
be delayed and executed just after the "soft reset", therefore resulting
in more than 1 VCPU running.

> 
> One disadvantage of the current approach is that diag 308 does not obey
> -no-reboot.

Both calls are used for kdump+kexec. "kdump on s390 uses a load normal
reset to bring the system in a defined state by doing a subsystem
reset", so like a "soft reboot". I don't think that we want to apply
"-no-reboot" here.

> 
> Paolo
> 


-- 

Thanks,

David / dhildenb