Re: [Qemu-devel] [PATCH v1 for-2.11 2/3] s390x/tcg: fix DIAG 308 with > 1 VCPU (single threaded TCG)

[David Hildenbrand <david@redhat.com>]

On 16.11.2017 18:37, Alex Bennée wrote:
> 
> David Hildenbrand <david@redhat.com> writes:
> 
>> Currently, single threaded TCG with > 1 VCPU gets stuck during IPL, when
>> the bios tries to switch to the loaded kernel via DIAG 308.
>>
>> pause_all_vcpus()/resume_all_vcpus() should not be called from a VCPU.
>>
>> And there is also no need for it. run_on_cpu() will make sure that the
>> CPUs exit KVM/TCG, where they get stopped. Once stopped, they will no
>> longer run.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>  target/s390x/diag.c | 4 ----
>>  1 file changed, 4 deletions(-)
>>
>> diff --git a/target/s390x/diag.c b/target/s390x/diag.c
>> index dbbb9e886f..52bc348808 100644
>> --- a/target/s390x/diag.c
>> +++ b/target/s390x/diag.c
>> @@ -27,7 +27,6 @@ static int modified_clear_reset(S390CPU *cpu)
>>      S390CPUClass *scc = S390_CPU_GET_CLASS(cpu);
>>      CPUState *t;
>>
>> -    pause_all_vcpus();
>>      cpu_synchronize_all_states();
>>      CPU_FOREACH(t) {
>>          run_on_cpu(t, s390_do_cpu_full_reset, RUN_ON_CPU_NULL);
> 
> I think you also need to fix the run_on_cpu to be a async_run_on_cpu as
> you would otherwise hang waiting for run_on_cpu to finish on a
> single-threaded TCG run (as you are in the only vCPU context).

No, it works just fine for single threaded TCG. run_on_cpu() can deal
with single threaded TCG just fine. (otherwise e.g. SIGP code also
wouldn't work)

In do_run_on_cpu, the following code always directly triggers for single
threaded tcg:

    if (qemu_cpu_is_self(cpu)) {

        func(cpu, data);

        return;

    }

> 
> If it is important that the source vCPU doesn't continue you can
> schedule it's work afterwards with a async_safe_run_on_cpu which will
> complete after all other vCPUs have executed their work.
> 

It is important. Introducing async helpers at a point where sync is
needed sounds strange.

Thanks!

>>
>> --
>> 2.13.6
> 
> 
> --
> Alex Bennée
> 


-- 

Thanks,

David / dhildenb