Re: [PATCH v7 27/28] iommu/tegra241-cmdqv: Add user-space use support

[Pranjal Shrivastava <praan@google.com>]

On Thu, Jul 03, 2025 at 02:55:32PM -0300, Jason Gunthorpe wrote:
> On Thu, Jul 03, 2025 at 02:46:03PM +0000, Pranjal Shrivastava wrote:
> 
> > Right.. I was however hoping we'd also trap commands like CMD_PRI_RESP
> > and CMD_RESUME...I'm not sure if they should be accelerated via CMDQV..
> > I guess I'll need to look and understand a little more if they are..
> 
> Right now these commands are not supported by vSMMUv3 in Linux.
> 
> They probably should be trapped, but completing a PRI (or resuming a
> stall which we will treat the same) will go through the PRI/page fault
> logic in iommufd not the cache invalidate.
> 

Ahh, thanks for this, that saved a lot of my time! And yes, I see some
functions in eventq.c calling the iopf_group_response which settles the
CMD_RESUME. So.. I assume these resume commands would be trapped and
*actually* executed through this or a similar path for vPRI. 

Meh, I had been putting off reading up the fault parts of iommufd, 
I guess I'll go through that too, now :) 

> > > The goal of the SMMU driver when it detects CMDQV support is to route
> > > all supported invalidations to CMDQV queues and then balance those
> > > queues across CPUs to reduce lock contention.
> > 
> > I see.. that makes sense.. so it's a relatively small gain (but a nice
> > one). Thanks for clarifying!
> 
> On bare metal the gain is small (due to locking and balancing), while
> on virtualization the gain is huge (due to no trapping).
> 

Ohh yes, I meant the bare metal gains here.. for virtualization, it's
definitely huge (as reported too).

> Regardless the SMMU driver uses cmdqv support if the HW says it is
> there.
>
> Jason
>

Thanks!
Praan