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 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.

Jan


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