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

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Message-ID: <aGSNmf5Q82xEbDpX@google.com>
Date: Wed, 2 Jul 2025 01:38:33 +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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	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
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	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 05:46:06PM -0700, Nicolin Chen wrote:
> On Wed, Jul 02, 2025 at 12:14:28AM +0000, Pranjal Shrivastava wrote:
> > 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.
> 
> VCMDQ only accelerates limited invalidation commands, not all of
> them: STE cache invalidation and CD cache invalidation commands
> still go down to that op.
> 

Right, I'm just saying the "other" non-accelerated commands that are
NOT invalidations also go down that op. So, if we add a comment, let's 
not call them "non-invalidation" commands.

> > 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),
> 
> I recall that FIXME is noted by Jason at that time. And it should
> be interpreted as "group by opcode", IIUIC.

I see.. I misunderstood that..

> 
> The thing is that for a host kernel that enabled in-kernel VCMDQs,
> those trapped user commands can be just issued to the smmu->cmdq
> or a vcmdq (picked via the get_secondary_cmdq impl_op).
> 

Ohh.. so maybe some sort of a load balancing thing?

> >    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 do something differently there, it could just do a one-time
> get_secondary_cmdq call to pick a in-kernel vcmdq over smmu->cmdq
> to fill in all the trapped commands.
> 

Alright.

> And this is not related to this series at all.

Agreed, sorry for the confusion then.. I thought that the "type" meant
the queue type.. I guess it's all done then. I have no further questions

Thanks for the clarification!

> 
> Nicolin

Praan