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

[Thomas Gleixner]

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?

Thanks,

        tglx