Re: [Xen-devel] [PATCH 3/4] VMX: Add posted interrupt supporting

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 09/04/2013 10:00, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

>>>> Can't you use vlapic_set_vector() here (even if that means
>>>> looping over vectors individually rather than groups), to add the
>>>> necessary atomicity (I don't see how you avoid races with other
>>>> updates) and to avoid the ugly cast?
>> Which races?
> 
> vlapic.c also updates the APIC_IRR bit array, and hence (unless
> you can guarantee that now and forever such updates only
> happen when the subject vCPU is current, which I don't think is
> the case)

It's not the case. Which is one reason the IRR updates (and I think all
bitmap updates) in vlapic.c are atomic.

 -- Keir

> your non-atomic read-modify-write operation here can
> discard an update done in vlapic.c. And am I wrong in recalling
> that the CPU may actually also on its own update that bit array?



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