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Re: [Qemu-devel] [PATCH v1 for-2.11 2/3] s390x/tcg: fix DIAG 308 with >

From: David Hildenbrand
Subject: Re: [Qemu-devel] [PATCH v1 for-2.11 2/3] s390x/tcg: fix DIAG 308 with > 1 VCPU (single threaded TCG)
Date: Thu, 16 Nov 2017 22:42:41 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 16.11.2017 21:57, Christian Borntraeger wrote:
> Please change the subject. In busy times I tend to ignore tcg patches.
> This code is clearly kvm and tcg.
> So what about "s390x/diag:" as prefix?

Right, it was a fix for TCG, that's why I added TCG only. But it should
have been purely s390x or s390x/diag.

> On 11/16/2017 06:05 PM, David Hildenbrand wrote:
>> 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 <address@hidden>
>> ---
>>  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();
> I did this to prevent a "still running CPU to restart an already stopped one".
> Are we sure that this can not happen?

Interesting question. I thought it would not be a problem but the way
locking with run_on_cpu() is handled is tricky. Now I am worried about a
couple of deadlocks.

pause_all_vcpus() actually gives up the qemu_global_mutex, so anybody
waiting for that mutex can continue.

We have a deadlock if two CPUs at the same time would call
pause_all_vcpus(). E.g. two CPUs executing at the same time a DIAG 308

run_on_cpu temporarily gives up the qemu_global_mutex. If two CPUs call
a run_on_cpu at the same time against each other, we might also have a

Two CPUs simultaneously trying to send a SIGP START/STOP/RESTART cannot
run into a deadlock as they are protected by the SIGP mutex with a trylock.

So even with pause_all_vcpus() I think we have a problem if another CPU
sends a SIGP to the issuing DIAG CPU and expects the run_on_cpu to
trigger. Deadlock, but unlikely I assume?

Let's defer this patch for now, booting with 1 VCPU is not degraded and
it looked easier than it is.

Maybe fixing pause_all_vcpus() to work with more than one CPU in single
threaded TCG is an (easier) alternative and at least keeps the (tested)
state. 2.12 material.



David / dhildenb

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