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

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Message-ID: <aGRmaOORg-YDfncY@google.com>
Date: Tue, 1 Jul 2025 22:51:20 +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,
	bagasdotme@...il.com, robin.murphy@....com, joro@...tes.org,
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	shuah@...nel.org, jsnitsel@...hat.com, nathan@...nel.org,
	peterz@...radead.org, yi.l.liu@...el.com, mshavit@...gle.com,
	zhangzekun11@...wei.com, iommu@...ts.linux.dev,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
	linux-kselftest@...r.kernel.org, patches@...ts.linux.dev,
	mochs@...dia.com, alok.a.tiwari@...cle.com, vasant.hegde@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com
Subject: Re: [PATCH v7 27/28] iommu/tegra241-cmdqv: Add user-space use support

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?

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?

> Nicolin

Praan