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

["Alex Bennée" <alex.bennee@linaro.org>]

David Hildenbrand <david@redhat.com> writes:

> 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;
>
>     }

Sorry ignore what I said, qemu_cpu_is_self tests the underlying thread
not the vCPU which they all share.


--
Alex Bennée