Re: [PATCH v9 3/7] iommu/arm-smmu-v3: Introduce a per-domain arm_smmu_invs array

Nicolin Chen
2026-01-23T19:19:09Z

On Fri, Jan 23, 2026 at 03:16:54PM -0400, Jason Gunthorpe wrote:
> On Fri, Jan 23, 2026 at 09:56:01AM -0800, Nicolin Chen wrote:
> > 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.
> >
> > ..?
>
> It doesn't need to be a full atomic, you can use WRITE_ONCE/READ_ONCE
> instead for this case.
>
> The general argument of this scheme is it doesn't matter if the
> invalidation side is doing extra invalidation, that is supposed to
> always be safe except for ATS. For ATS we hold locks and then it
> doesn't to be atomic because of the lock.
>
> So I think Will has it right, just use READ_ONCE/WRITE_ONCE on a naked
> unsigned int.

OK. I'll change that.

Thanks
Nicolin