Re: [Xen-devel] [v4 11/17] vt-d: Add API to update IRTE when VT-d PI is used

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.07.15 at 17:55, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 23/07/15 16:52, Jan Beulich wrote:
>>>>> On 23.07.15 at 15:51, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 23/07/15 12:35, Feng Wu wrote:
>>>> +    GET_IREMAP_ENTRY(ir_ctrl->iremap_maddr, remap_index, iremap_entries, 
>>>> p);
>>>> +
>>>> +    old_ire = new_ire = *p;
>>>> +
>>>> +    /* Setup/Update interrupt remapping table entry. */
>>>> +    setup_posted_irte(&new_ire, pi_desc, gvec);
>>>> +    ret = cmpxchg16b(p, &old_ire, &new_ire);
>>>> +
>>>> +    ASSERT(ret == *(__uint128_t *)&old_ire);
>>> This cannot be correct.  Either the cmpxchg() is required and you must
>>> cope with it failing, or the cmpxchg() is not required and this should
>>> be a plain write.
>> Not exactly: The cmpxchg() is required for this to be an atomic
>> 128-bit write. And hence I would view the ASSERT() as
>> appropriate - it validates that the entry didn't change behind our
>> back.
> 
> But p is an active descriptor, which means hardware is liable to change
> it behind our back.

I inquired about this on the previous round and was told
hardware doesn't alter the descriptor. Comparing this with
the spec, I couldn't spot any fields that I would suspect
getting written. Which fields do you have in mind?

Jan


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