linux-kernel - Re: [PATCH RFCv1 3/3] iommu/arm-smmu-v3: Allow ATS to be always on

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Message-ID: <aXe1J6Rygrvoz/Wo@Asurada-Nvidia>
Date: Mon, 26 Jan 2026 10:40:39 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Will Deacon <will@...nel.org>, <robin.murphy@....com>,
	<bhelgaas@...gle.com>, <joro@...tes.org>, <praan@...gle.com>,
	<baolu.lu@...ux.intel.com>, <kevin.tian@...el.com>,
	<miko.lenczewski@....com>, <linux-arm-kernel@...ts.infradead.org>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>,
	<linux-pci@...r.kernel.org>
Subject: Re: [PATCH RFCv1 3/3] iommu/arm-smmu-v3: Allow ATS to be always on

On Mon, Jan 26, 2026 at 01:20:20PM -0400, Jason Gunthorpe wrote:
> On Mon, Jan 26, 2026 at 12:39:50PM +0000, Will Deacon wrote:
> > > +	ret = arm_smmu_alloc_cd_tables(master);
> > > +	if (ret)
> > > +		return ret;
> > 
> > Were do you allocate the second level entry for ssid 0 if we're using
> > 2-level cd tables?
> 
> I don't think we need to. The entire design here has a non-valid CD entry
> for SSID 0.

Hmm, whether we allocate a 2-level cd table would actually depend on
the "1 << cd_table->s1cdmax" v.s. CTXDESC_L2_ENTRIES, right?

If the device supports PASID and s1cdmax is large, we should prepare
a 2-level cd tables, even if only SSID0 is used at this moment since
we have to support !0 pasids via potential SVA domains.

In all Other cases, we would prepare a linear one.

Nicolin