Re: [RFC PATCH 1/1] xen: Use a global mapping for runstate

[Julien Grall <julien@xen.org>]

Hi,

On 28/05/2020 18:19, Bertrand Marquis wrote:
>>> +
>>> +    return 0;
>>> +}
>>> +
>>> int domain_kill(struct domain *d)
>>> {
>>>      int rc = 0;
>>> @@ -727,7 +788,10 @@ int domain_kill(struct domain *d)
>>>          if ( cpupool_move_domain(d, cpupool0) )
>>>              return -ERESTART;
>>>          for_each_vcpu ( d, v )
>>> +        {
>>> +            unmap_runstate_area(v, 0);
>>
>> Why is it not appropriate here to hold the lock? It might not be
>> technically needed, but still should work?
> 
> In a killing scenario you might stop a core while it was rescheduling.
> Couldn’t a core be killed using a cross core interrupt ?

You can't stop a vCPU in the middle of the context switch. The vCPU can 
only be scheduled out at specific point in Xen.

> If this is the case then I would need to do masked interrupt locking sections to prevent that case ?

At the beginning of the function domain_kill() the domain will be paused 
(see domain_pause()). After this steps none of the vCPUs will be running 
or be scheduled.

You should technically use the lock everywhere to avoid static analyzer 
throwing a warning because you access variable with and without the 
lock. A comment would at least be useful in the code if we go ahead with 
this.

As an aside, I think you want the lock to always disable the interrupts 
otherwise check_lock() (this can be enabled with CONFIG_DEBUG_LOCKS only 
on x86 though) will shout at you because your lock can be taken in both 
IRQ-safe and IRQ-unsafe context.

Cheers,

-- 
Julien Grall