linux-kernel - Re: [PATCH v4 11/23] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE

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Message-ID: <aCn6hrev3k7A1Q1E@Asurada-Nvidia>
Date: Sun, 18 May 2025 08:19:34 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
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Subject: Re: [PATCH v4 11/23] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE_ALLOC
 ioctl

Hi Kevin,

On Thu, May 15, 2025 at 09:05:11PM -0700, Nicolin Chen wrote:
> On Fri, May 16, 2025 at 03:52:16AM +0000, Tian, Kevin wrote:
> > But hey, we are already adding various restrictions to the uAPI
> > about dependency, contiguity, etc. which the VMM should conform
> > to. What hurts if we further say that the VMM should allocate
> > virtual index in an ascending order along with hw queue allocation?
> 
> You mean adding another flag to manage the dependency in the core,
> right?
> 
> I talked with Jason offline when adding that depend API. He didn't
> want it to be in the core, saying that is a driver thing.
> 
> But that was before we added pin and contiguity, which he doesn't
> really enjoy being in the core either.
> 
> So, yea, I think you have a point here..

It seems Jason is out of office. And in the last sync w.r.t this,
he thinks that this ascending order stuff is too unique/weird to
make sense as a feature in the core, that there would be unlikely
a second HW wanting this..

I think that's a valid point too. The pin/contiguity requirement
at least serves for HW that reads in physical address space, and
it could result in a slightly faster memory access since it does
not need a translation, which though gives software some trouble
yet still makes sense IMHO.

So, in v5, I kept the dependency APIs rather than moving to the
core. I think we can move to the core later if we see another HW
doing the same thing.

Thanks
Nicolin