linux-kernel - Re: [PATCH v9 3/7] iommu/arm-smmu-v3: Introduce a per-domain arm_smmu

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Message-ID: <aXO2MY6gzs/lxT7S@Asurada-Nvidia>
Date: Fri, 23 Jan 2026 09:56:01 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Will Deacon <will@...nel.org>
CC: <jean-philippe@...aro.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<jgg@...dia.com>, <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 v9 3/7] iommu/arm-smmu-v3: Introduce a per-domain
 arm_smmu_invs array

On Fri, Jan 23, 2026 at 05:51:52PM +0000, Will Deacon wrote:
> On Fri, Jan 23, 2026 at 09:35:58AM -0800, Nicolin Chen wrote:
> > On Fri, Jan 23, 2026 at 05:03:10PM +0000, Will Deacon wrote:
> > > On Fri, Dec 19, 2025 at 12:11:25PM -0800, Nicolin Chen wrote:
> > > > +struct arm_smmu_inv {
> > > > +	struct arm_smmu_device *smmu;
> > > > +	u8 type;
> > > > +	u8 size_opcode;
> > > > +	u8 nsize_opcode;
> > > > +	u32 id; /* ASID or VMID or SID */
> > > > +	union {
> > > > +		size_t pgsize; /* ARM_SMMU_FEAT_RANGE_INV */
> > > > +		u32 ssid; /* INV_TYPE_ATS */
> > > > +	};
> > > > +
> > > > +	refcount_t users; /* users=0 to mark as a trash to be purged */
> > > 
> > > The refcount_t API uses atomics with barrier semantics. Do we actually
> > > need those properties when updating the refcounts here? The ASID lock
> > > gives us pretty strong serialisation even after this patch series and
> > > we rely heavily on that.
> > 
> > But we can't use that mutex in the invalidation function that
> > might be an IRQ context?
> 
> My question, really, is why do you need the atomic properties in this patch
> series? It just looks like overhead at the moment.

Hmm, shouldn't it be atomic, since..

(might be IRQ, no mutex) __arm_smmu_domain_inv_range() reads it.
(mutex protected) arm_smmu_attach_dev() writes it.

..?

Nicolin