linux-kernel - Re: [PATCH v5 4/7] iommu/arm-smmu-v3: Pre-allocate a per-master invalidation array

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Message-ID: <aSTqw4Doipb+0AfP@Asurada-Nvidia>
Date: Mon, 24 Nov 2025 15:31:15 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Will Deacon <will@...nel.org>, <jean-philippe@...aro.org>,
	<robin.murphy@....com>, <joro@...tes.org>, <balbirs@...dia.com>,
	<miko.lenczewski@....com>, <peterz@...radead.org>, <kevin.tian@...el.com>,
	<praan@...gle.com>, <linux-arm-kernel@...ts.infradead.org>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 4/7] iommu/arm-smmu-v3: Pre-allocate a per-master
 invalidation array

On Mon, Nov 24, 2025 at 07:08:45PM -0400, Jason Gunthorpe wrote:
> On Mon, Nov 24, 2025 at 02:43:58PM -0800, Nicolin Chen wrote:
> > On Mon, Nov 24, 2025 at 09:42:55PM +0000, Will Deacon wrote:
> > > On Sat, Nov 08, 2025 at 12:08:05AM -0800, Nicolin Chen wrote:
> > > > +		/* Put the ids into order for sorted to_merge/to_unref arrays */
> > > > +		sort_nonatomic(fwspec->ids, fwspec->num_ids,
> > > > +			       sizeof(fwspec->ids[0]), arm_smmu_ids_cmp, NULL);
> > > > +		/* ATS case adds num_ids of entries, on top of the base case */
> > > > +		master->build_invs = arm_smmu_invs_alloc(2 + fwspec->num_ids);
> > > 
> > > Although I can't point at a specific issue here, I'm nervous about mutating
> > > the 'fwspec->ids' array from within the driver, The array isn't allocated
> > > or populated directly by the driver and so I don't think we really have any
> > > business sorting it. Could we hack iommu_fwspec_add_ids() to keep the array
> > > ordered instead?
> > 
> > Yea, I think it makes sense to do it in the core, once we have the
> > data structure provided by the core as well.
> 
> I would be more worried about sorting it everywhere for every
> driver. I feel confident SMMUv3 doesn't use it, but something really
> old and embedded focused like tegra or omap, IDK.
> 
> So I wouldn't propose to change iommu_fwspec_add_ids().

Perhaps a different helper iommu_fwspec_add_ids_sorted()? Drivers
can choose to use the sorted version, when they want to implement
the invalidation array. And SMMU can be the first only caller.

> If you want to be conservative then the thing to do is sort the
> master->streams that arm_smmu_insert_master() copies the fwspec
> into. It just has to be sorted prior to feeding it into the rbtree.
> 
> Then consistently use master->streams as the sorted list.

How about kmemdup() an local id array to bridge betwen fwspec->ids
and rbtree?

Thanks
Nicolin