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

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Message-ID: <aGR55PUBnwb8qT8U@google.com>
Date: Wed, 2 Jul 2025 00:14:28 +0000
From: Pranjal Shrivastava <praan@...gle.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...dia.com, kevin.tian@...el.com, corbet@....net, will@...nel.org,
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Subject: Re: [PATCH v7 27/28] iommu/tegra241-cmdqv: Add user-space use support

On Tue, Jul 01, 2025 at 04:01:34PM -0700, Nicolin Chen wrote:
> On Tue, Jul 01, 2025 at 10:51:20PM +0000, Pranjal Shrivastava wrote:
> > On Tue, Jul 01, 2025 at 03:07:57PM -0700, Nicolin Chen wrote:
> > > On Tue, Jul 01, 2025 at 08:43:30PM +0000, Pranjal Shrivastava wrote:
> > > > On Tue, Jul 01, 2025 at 01:23:17PM -0700, Nicolin Chen wrote:
> > > > > Or perhaps calling them "non-accelerated commands" would be nicer.
> > > > 
> > > > Uhh okay, so there'll be a separate driver in the VM issuing invalidation
> > > > commands directly to the CMDQV thus we don't see any of it's part here?
> > > 
> > > That's how it works. VM must run a guest-level VCMDQ driver that
> > > separates accelerated and non-accelerated commands as it already
> > > does:
> > > 
> > >     accelerated commands => VCMDQ (HW)
> > > non-accelerated commands => SMMU CMDQ (SW) =iommufd=> SMMU CMDQ (HW)
> > > 
> > 
> > Right exactly what got me confused. I was assuming the same CMDQV driver
> > would run in the Guest kernel but seems like there's another driver for
> > the Guest that's not in tree yet or maybe is a purely user-space thing?
> 
> It's the same tegra241-cmdqv.c in the kernel, which is already
> a part of mainline Linux. Both host and guest run the same copy
> of software. The host kernel just has the user VINTF part (via
> iommufd) additional to what the guest already has.
> 
> > And the weird part was that "invalidation" commands are accelerated but
> > we use the .cache_invalidate viommu op for `non-invalidation` commands.
> > But I guess what you meant there could be non-accelerated invalidation 
> > commands (maybe something stage 2 TLBIs?) which would go through the 
> > .cache_invalidate op, right?
> 
> I am talking about this:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c?h=v6.16-rc4#n305
> 
> Those commands returned "false" will be issued to smmu->cmdq in a
> guest VM, which will be trapped by VMM as a standard SMMU nesting
> and will be further forwarded via iommufd to the host kernel that
> will invoke this cache_invalidate op in the arm-smmu-v3 driver.
> 
> Those commands returned "true" will be issued to vcmdq->cmdq that
> is HW-accelerated queue (setup by VMM via iommufd's hw_queue/mmap).


Right, this brings me back to my original understanding, the arm-smmu-v3
driver checks for "supported commands" and figures out which queue shall
they be issued to.. now there are commands which are "non-invalidation"
commands which are non-acclerated like CMD_PRI_RESP, which would be
issued through the trap => .cache_invalidate path. 

Thus, coming back to the two initial points:

1) Issuing "non-invalidation" commands through .cache_invalidate could
   be confusing, I'm not asking to change the op name here, but if we
   plan to label it, let's label them as "Trapped commands" OR
   "non-accelerated" commands as you suggested.

2) The "FIXME" confusion: The comment in arm_vsmmu_cache_invalidate
   mentions we'd like to "fix" the issuing of commands through the main
   cmdq and instead like to group by "type", if that "type" is the queue
   type (which I assume it is because IOMMU_TYPE has to be arm-smmu-v3),
   what do we plan to do differently there, given that the op is only
   for trapped commands *have* to go through the main CMDQ?

   If we were planning to do something based on the queue type, I was
   hoping for it to be addressed in this series as we've introduced the
   Tegra CMDQ type. 

That's all I wanted to say, sorry for if this was confusing.

> 
> Nicolin

Thanks,
Praan