>>> On 15.07.15 at 10:38, <feng...@intel.com> wrote:
>
>> -----Original Message-----
>> From: Jan Beulich [mailto:jbeul...@suse.com]
>> Sent: Wednesday, July 15, 2015 4:25 PM
>> To: Wu, Feng
>> Cc: andrew.coop...@citrix.com; george.dun...@eu.citrix.com; Tian, Kevin;
>> Zhang, Yang Z; xen-devel@lists.xen.org; k...@xen.org
>> Subject: RE: [v3 10/15] vt-d: Add API to update IRTE when VT-d PI is used
>>
>> >>> On 15.07.15 at 08:04, <feng...@intel.com> wrote:
>> >> From: Jan Beulich [mailto:jbeul...@suse.com]
>> >> Sent: Friday, July 10, 2015 10:02 PM
>> >> I'm particularly worried by the call to acpi_find_matched_drhd_unit()
>> >> - is it maybe worth storing the iommu pointer in struct msi_desc?
>> >
>> > I think it worth, Like Andrew also mentioned this point before. I tend
>> > to make this a independent work and do it later, since the 4.6 release
>> > is coming, I am still try my best to target it. Could you please share
>> > your concern here, performance? Or other things? Thanks!
>>
>> Interrupt latency in particular.
>
> This update IRTE operation is not so frequently. It only happens in few
> times,
> especially in the initialization phase of the guest. And even the guest set
> the affinity, in the MSI/MSIx configuration doesn't change, QEMU will not
> ask Xen to update it.
When the guest sets the affinity, the MSI{,-X} configuration is
rather likely to change (at least for Linux guests).
>> There are two possible scenarios:
>>
>> 1) There are bits that can be updated behind the back of the code
>> here. In that case you need to loop, and each iteration of the loop
>> needs to re-fetch the current value (not doing so would make the
>> loop infinite).
>
> Oh, yes, I think I made a mistake here, it is too hastily these days,
> Sorry for that! I think I need do it like this:
>
> do {
> new_ire = *p;
>
> /* Setup/Update interrupt remapping table entry. */
> setup_posted_irte(&new_ire, pi_desc, gvec);
>
> old_ire = *(uint128_t *)p;
> ret = cmpxchg16b(p, &old_ire, &new_ire);
> } while ( memcmp(&ret, &old_ire, sizeof(old_ire)) );
So since you put this in a loop again, would you mind pointing out
which bits can get modified behind our back?
Jan
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