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

[Dave Jiang]

On 12/1/2021 1:25 PM, Thomas Gleixner wrote:
> On Wed, Dec 01 2021 at 11:47, Dave Jiang wrote:
> > On 12/1/2021 11:41 AM, Thomas Gleixner wrote:
> > > > Hi Thomas. This is actually the IDXD usage for a mediated device passed
> > > > to a guest kernel when we plumb the pass through of IMS to the guest
> > > > rather than doing previous implementation of having a MSIX vector on
> > > > guest backed by IMS.
> > > Which makes a lot of sense.
> > >
> > > > The control block for the mediated device is emulated and therefore an
> > > > emulated MSIX vector will be surfaced as vector 0. However the queues
> > > > will backed by IMS vectors. So we end up needing MSIX and IMS coexist
> > > > running on the guest kernel for the same device.
> > > Why? What's wrong with using straight MSI-X for all of them?
> > 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?


> Thanks,
>
>          tglx