Re: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry

[Nadav Amit]

> On Oct 14, 2016, at 11:56 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>>> 
>>> 	for (i = 0; i <= 7; i++) {
>>> -		pir_val = xchg(&pir[i], 0);
>>> -		if (pir_val)
>>> +		pir_val = READ_ONCE(pir[i]);
>> 
>> Out of curiosity, do you really need this READ_ONCE?
> 
> The answer can only be "depends on the compiler's whims". :)
> If you think of READ_ONCE as a C11 relaxed atomic load, then yes.

Hm.. So the idea is to make the code "race-freeâ in the sense
that every concurrent memory access is done using READ_ONCE/WRITE_ONCE?

If that is the case, I think there are many other cases that need to be
changed, for example apic->irr_pending and vcpu->arch.pv.pv_unhalted.