RE: [patch 21/32] NTB/msi: Convert to msi_on_each_desc()

[Tian, Kevin]

> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Sent: Thursday, December 2, 2021 5:45 AM
> 
> On Wed, Dec 01 2021 at 14:21, Dave Jiang wrote:
> > On 12/1/2021 1:25 PM, Thomas Gleixner wrote:
> >>> The hardware implementation does not have enough MSIX vectors for
> >>> guests. There are only 9 MSIX vectors total (8 for queues) and 2048 IMS
> >>> vectors. So if we are to do MSI-X for all of them, then we need to do
> >>> the IMS backed MSIX scheme rather than passthrough IMS to guests.
> >> Confused. Are you talking about passing a full IDXD device to the guest
> >> or about passing a carved out subdevice, aka. queue?
> >
> > I'm talking about carving out a subdevice. I had the impression of you
> > wanting IMS passed through for all variations. But it sounds like for a
> > sub-device, you are ok with the implementation of MSIX backed by IMS?
> 
> I don't see anything wrong with that. A subdevice is it's own entity and
> VFIO can chose the most conveniant representation of it to the guest
> obviously.
> 
> How that is backed on the host does not really matter. You can expose
> MSI-X to the guest with a INTx backing as well.
> 

Agree with this point. How the interrupts are represented to the guest
is orthogonal to how the backend resource is allocated. Physically MSI-X 
and IMS can be enabled simultaneously on an IDXD device. Once 
dynamic allocation is allowed for both, either one can be allocated for
a subdevice (with only difference on supported #subdevices). 

When an interrupt resource is exposed to the guest with the same type 
(e.g. MSI-on-MSI or IMS-on-IMS), it can be also passed through to the 
guest as long as a hypercall machinery is in place to get addr/data pair 
from the host (as you suggested earlier).

Thanks
Kevin