Re: [PATCH v9 6/7] iommu/arm-smmu-v3: Add arm_smmu_invs based arm_smmu_domain_inv_range()

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Jan 26, 2026 at 04:02:19PM +0000, Will Deacon wrote:
> On Mon, Jan 26, 2026 at 11:20:19AM -0400, Jason Gunthorpe wrote:
> > On Mon, Jan 26, 2026 at 01:01:16PM +0000, Will Deacon wrote:
> > > On Fri, Jan 23, 2026 at 04:03:27PM -0400, Jason Gunthorpe wrote:
> > > > 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();

> If we do that, can we drop the smp_mb()s from
> arm_smmu_install_{old,new}_domain_invs()?

I suppose so, but domain attach isn't a performance path so it depends
on your preference for strict pairing of barriers. Currently the two
smp_mbs() are paired. Can we reliably pair smp_mb() with dma_wmb()?
Are you happy with that clarity?

My view is attach isn't a performance path, so having extra barriers
is fine if it helps understandability.

Jason