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

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Message-ID: <aXO1OLrOsCkep3V8@willie-the-truck>
Date: Fri, 23 Jan 2026 17:51:52 +0000
From: Will Deacon <will@...nel.org>
To: Nicolin Chen <nicolinc@...dia.com>
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 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.

Will