linux-kernel - Re: [PATCH rfcv1 5/8] iommu/arm-smmu-v3: Pre-allocate a per-master invalidation array

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Message-ID: <20250826195641.GB2151485@nvidia.com>
Date: Tue, 26 Aug 2025 16:56:41 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...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, zhangzekun11@...wei.com,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH rfcv1 5/8] iommu/arm-smmu-v3: Pre-allocate a per-master
 invalidation array

On Wed, Aug 13, 2025 at 06:25:36PM -0700, Nicolin Chen wrote:
> @@ -3730,6 +3731,13 @@ static int arm_smmu_insert_master(struct arm_smmu_device *smmu,
>  		return -ENOMEM;
>  	master->num_streams = fwspec->num_ids;
>  
> +	/* Max possible num_invs: two for ASID/VMIDs and num_ats for ATC_INVs */
> +	master->invs = arm_smmu_invs_alloc(2 + num_ats);
> +	if (IS_ERR(master->invs)) {
> +		kfree(master->streams);
> +		return PTR_ERR(master->invs);
> +	}

This seems like a nice solution, but I would add a comment here that
it is locked by the group mutex, and check if ATS is supported:

	/*
	 * Scratch memory to build the per-domain invalidation list. locked by
	 * the group_mutex. Max possible num_invs: two for ASID/VMIDs and
	 * num_streams for ATC_INVs
	 */
	if (dev_is_pci(master->dev) &&
	    pci_ats_supported(to_pci_dev(master->dev)))
		master->invs = arm_smmu_invs_alloc(2 + master->num_streams);
	else
		master->invs = arm_smmu_invs_alloc(2);

And probably rename it scratch_invs or something to indicate it is
temporary memory.

I'm not sure there is any case where fwspec->num_ids >1 &&
ats_supported, or at least is should be really rare.

Jason