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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.04.13 at 10:53, "Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx> wrote:
> Zhang, Yang Z wrote on 2013-04-09:
>> Jan Beulich wrote on 2013-04-09:
>>>>>> On 09.04.13 at 08:01, Yang Zhang <yang.z.zhang@xxxxxxxxx> wrote:
>>>> +    {
>>>> +        val = pi_get_pir(&v->arch.hvm_vmx.pi_desc, group);
>>>> +        offset = APIC_IRR + 0x10 * group;
>>>> +        *((uint32_t *)(&vlapic->regs->data[offset])) |= val;
>>> 
>>> 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) 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?

Jan


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