Re: [Qemu-devel] [RFC PATCH v4 13/23] cpus: only take BQL for sleeping threads

[Pavel Dovgalyuk]

Pavel Dovgalyuk


> -----Original Message-----
> From: Pavel Dovgalyuk [mailto:address@hidden
> Sent: Friday, January 19, 2018 3:37 PM
> To: 'Paolo Bonzini'; 'Pavel Dovgalyuk'; address@hidden
> Cc: address@hidden; address@hidden; address@hidden; address@hidden;
> address@hidden; address@hidden; address@hidden; address@hidden;
> address@hidden; address@hidden
> Subject: RE: [RFC PATCH v4 13/23] cpus: only take BQL for sleeping threads
> 
> > From: Paolo Bonzini [mailto:address@hidden
> > Sent: Friday, January 19, 2018 3:26 PM
> > To: Pavel Dovgalyuk; 'Pavel Dovgalyuk'; address@hidden
> > Cc: address@hidden; address@hidden; address@hidden; address@hidden;
> > address@hidden; address@hidden; address@hidden; address@hidden;
> > address@hidden; address@hidden
> > Subject: Re: [RFC PATCH v4 13/23] cpus: only take BQL for sleeping threads
> >
> > On 19/01/2018 13:25, Pavel Dovgalyuk wrote:
> > >>> It means, that I'll have to fix all the has_work function to avoid 
> > >>> races,
> > >>> because x86_cpu_has_work may have them?
> > >> Why only x86_cpu_has_work?
> > >>
> > >> Even reading cs->interrupt_request outside the mutex is unsafe.
> > > All the vcpu function that access interrupt controller or peripheral 
> > > state may be unsafe?
> > > How can it work safely then?
> >
> > They do it inside the big QEMU lock.
> 
> Right. Without these patches.

Ah, I forgot about unlocking in tcg_cpu_exec.
Now I see, that vcpu is taking the lock when trying to access the peripherals.

Therefore, I have to fix all *_cpu_has_work, right?

> They are within the replay lock. And BQL is not covering vcpu execution with 
> these patches.
> Therefore RR will be ok and regular execution may encounter races?
> It means that I missed something in Alex ideas, because he prepared the 
> initial patches.
> 
> > But here you're calling cpu_has_work (via all_cpu_threads_idle) outside the 
> > lock.
> 
> Yes, I see, but what we have to do?


Pavel Dovgalyuk