Re: [patch 3/4] QEMU/KVM: ignore SIG_IPI signals in userspace

[Avi Kivity <avi@qumranet.com>]

Marcelo Tosatti wrote:
> On Sun, Apr 06, 2008 at 12:15:07PM +0300, Avi Kivity wrote:
>   
>> Marcelo Tosatti wrote:
>>     
>>> Otherwise a signal can be received in userspace and a vcpu goes back
>>> to the kernel while it should stay still.
>>>
>>> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>>>
>>> Index: kvm-userspace.io/qemu/qemu-kvm.c
>>> ===================================================================
>>> --- kvm-userspace.io.orig/qemu/qemu-kvm.c
>>> +++ kvm-userspace.io/qemu/qemu-kvm.c
>>> @@ -350,7 +350,6 @@ static void *ap_main_loop(void *_env)
>>>     vcpu->env = env;
>>>     vcpu->env->thread_id = kvm_get_thread_id();
>>>     sigfillset(&signals);
>>> -    sigdelset(&signals, SIG_IPI);
>>>     sigprocmask(SIG_BLOCK, &signals, NULL);
>>>     kvm_create_vcpu(kvm_context, env->cpu_index);
>>>     kvm_qemu_init_env(env);
>>>
>>>  
>>>       
>> Does this work with -no-kvm-irqchip?
>>     
>
> Yes. SIG_IPI was blocked before the IO thread.
>
>   

Okay (of course; it is blocked, but still dequeued by sigtimedwait).

>> I think we need to fix the kernel to handle random signals.  Otherwise 
>> even attaching a debugger can change guest behavior (I think).
>>     
>
> Well ptrace forces signals so SIGSTOP is delivered even though the child
> has blocked them.
>
> Attaching a debugger does change behaviour since SIGSTOP will send a
> vcpu back to userspace.
>
> Can you be more specific?
>
>   

I misunderstood. I thought something about a spurious signal being 
received in the kernel, dropping it out of hlt state, and confusing the 
guest; but the real issue is the signal getting lost completely if it is 
delivered to userspace instead of sigtimedwait(), which this patch fixes.

-- 
Any sufficiently difficult bug is indistinguishable from a feature.


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