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

Will Deacon
2026-01-26T18:56:40Z

On Mon, Jan 26, 2026 at 12:09:40PM -0400, Jason Gunthorpe wrote:
> 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?

Yeah, I think that's ok.

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

I think that the more barriers we have, the harder the code is to
understand so I would prefer just to have the smp_mb() for the
write->read case in arm_smmu_domain_inv_range() along with a comment
explaining why it's needed.

I think that also means that my concern about the old comments on the
other patch largely disappears.

Will