linux-kernel - Re: [PATCH rfcv2 5/8] iommu/arm-smmu-v3: Pre-allocate a per-master invalidation array

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Message-ID: <aNrZ8z92CXs+J1pH@Asurada-Nvidia>
Date: Mon, 29 Sep 2025 12:11:47 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <will@...nel.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<jean-philippe@...aro.org>, <miko.lenczewski@....com>, <balbirs@...dia.com>,
	<peterz@...radead.org>, <smostafa@...gle.com>, <kevin.tian@...el.com>,
	<praan@...gle.com>, <linux-arm-kernel@...ts.infradead.org>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH rfcv2 5/8] iommu/arm-smmu-v3: Pre-allocate a per-master
 invalidation array

On Wed, Sep 24, 2025 at 06:32:30PM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 08, 2025 at 04:26:59PM -0700, Nicolin Chen wrote:
> > +	/* Base case has 1 ASID or 1~2 VMIDs. ATS case adds num_ids */
> > +	if (!ats_supported)
> > +		master->build_invs = arm_smmu_invs_alloc(2);
> > +	else
> > +		master->build_invs = arm_smmu_invs_alloc(2 + fwspec->num_ids);
> > +	if (IS_ERR(master->build_invs)) {
> > +		kfree(master->streams);
> > +		return PTR_ERR(master->build_invs);
> > +	}
> > +
> > +	/* Put the ids into order for a sorted to_merge or to_unref array */
> > +	sort_nonatomic(fwspec->ids, fwspec->num_ids, sizeof(fwspec->ids[0]),
> > +		       arm_smmu_ids_cmp, NULL);
> 
> The sort could be moved under the above !ats_supported, a little more
> insurance in case something is inspecting the ids.

You mean this:
----------------------------------------------------------------
@@ -4080,19 +4080,19 @@ static int arm_smmu_insert_master(struct arm_smmu_device *smmu,
        master->num_streams = fwspec->num_ids;

        /* Base case has 1 ASID or 1~2 VMIDs. ATS case adds num_ids */
-       if (!ats_supported)
+       if (!ats_supported) {
                master->build_invs = arm_smmu_invs_alloc(2);
-       else
+       } else {
+               /* Put the ids into order for a sorted to_merge or to_unref array */
+               sort_nonatomic(fwspec->ids, fwspec->num_ids, sizeof(fwspec->ids[0]),
+                              arm_smmu_ids_cmp, NULL);
                master->build_invs = arm_smmu_invs_alloc(2 + fwspec->num_ids);
+       }
        if (IS_ERR(master->build_invs)) {
                kfree(master->streams);
                return PTR_ERR(master->build_invs);
        }

-       /* Put the ids into order for a sorted to_merge or to_unref array */
-       sort_nonatomic(fwspec->ids, fwspec->num_ids, sizeof(fwspec->ids[0]),
-                      arm_smmu_ids_cmp, NULL);
-
        mutex_lock(&smmu->streams_mutex);
        for (i = 0; i < fwspec->num_ids; i++) {
                struct arm_smmu_stream *new_stream = &master->streams[i];
----------------------------------------------------------------
?

Hmm, I am not sure how it insures against anything concurrent.

Maybe we should sort it in arm_smmu_of_xlate() each time when
adding a new ID? Or iommu_fwspec_add_ids() itself could sort,
since we are thinking of generalizing this array in the core?

Thanks
Nicolin