linux-kernel - Re: [PATCH v9 6/7] iommu/arm-smmu-v3: Add arm_smmu_invs based arm_smmu_domain_inv

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Message-ID: <aXOzJwQJ7z8qKcEH@Asurada-Nvidia>
Date: Fri, 23 Jan 2026 09:43:03 -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 6/7] iommu/arm-smmu-v3: Add arm_smmu_invs based
 arm_smmu_domain_inv_range()

On Fri, Jan 23, 2026 at 05:10:52PM +0000, Will Deacon wrote:
> On Fri, Jan 23, 2026 at 05:05:31PM +0000, Will Deacon wrote:
> > On Fri, Dec 19, 2025 at 12:11:28PM -0800, Nicolin Chen wrote:
> > > +	/*
> > > +	 * We are committed to updating the STE. Ensure the invalidation array
> > > +	 * is visible to concurrent map/unmap threads, and acquire any racing
> > > +	 * IOPTE updates.
> > > +	 *
> > > +	 *  [CPU0]                        | [CPU1]
> > > +	 *                                |
> > > +	 *  change IOPTEs and TLB flush:  |
> > > +	 *  arm_smmu_domain_inv_range() { | arm_smmu_install_old_domain_invs {
> > > +	 *    ...                         |   rcu_assign_pointer(new_invs);
> > > +	 *    smp_mb(); // ensure IOPTEs  |   smp_mb(); // ensure new_invs
> > > +	 *    ...                         |   kfree_rcu(old_invs, rcu);
> > > +	 *    // load invalidation array  | }
> > > +	 *    invs = rcu_dereference();   | arm_smmu_install_ste_for_dev {
> > > +	 *                                |   STE = TTB0 // read new IOPTEs
> > > +	 */
> > > +	smp_mb();
> > 
> > I don't think we need to duplicate this comment three times, you can just
> > refer to the first function (e.g. "See ordering comment in
> > arm_smmu_domain_inv_range()").

I'll drop the duplicates.

> > However, isn't the comment above misleading for this case?
> > arm_smmu_install_old_domain_invs() has the sequencing the other way
> > around on CPU 1: we should update the STE first.

Ah, that's true. It installs new domain invs first then ste, and
lastly updates the old domain invs.

> I also think we probably want a dma_mb() instead of an smp_mb() for all
> of these examples? It won't make any practical difference but I think it
> helps readability given that one of the readers is the PTW.

OK. I'll change to dma_mb().

Nicolin