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

Dave Jiang
2021-12-01T21:21:23Z

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
>