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

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Message-ID: <aGSBTpY0nkdp2TTL@Asurada-Nvidia>
Date: Tue, 1 Jul 2025 17:46:06 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Pranjal Shrivastava <praan@...gle.com>
CC: <jgg@...dia.com>, <kevin.tian@...el.com>, <corbet@....net>,
	<will@...nel.org>, <bagasdotme@...il.com>, <robin.murphy@....com>,
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Subject: Re: [PATCH v7 27/28] iommu/tegra241-cmdqv: Add user-space use support

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.

> 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.

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).

>    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.

And this is not related to this series at all.

Nicolin