Messages in this thread | | | Date | Wed, 1 Dec 2021 11:47:35 -0700 | Subject | Re: [patch 21/32] NTB/msi: Convert to msi_on_each_desc() | From | Dave Jiang <> |
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On 12/1/2021 11:41 AM, Thomas Gleixner wrote: > Dave, > > please trim your replies. > > On Wed, Dec 01 2021 at 09:28, Dave Jiang wrote: > >> On 12/1/2021 3:16 AM, Thomas Gleixner wrote: >>> Jason, >>> >>> CC+ IOMMU folks >>> >>> On Tue, Nov 30 2021 at 20:17, Jason Gunthorpe wrote: >>>> On Tue, Nov 30, 2021 at 10:23:16PM +0100, Thomas Gleixner wrote: >>> Though I fear there is also a use case for MSI-X and IMS tied to the >>> same device. That network card you are talking about might end up using >>> MSI-X for a control block and then IMS for the actual network queues >>> when it is used as physical function device as a whole, but that's >>> conceptually a different case. >> 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.
> > Thanks, > > tglx >
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