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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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