[PATCH 11/27] iommu/arm-smmu-v3: Lift CD programming out of the SVA notifier code

[Michael Shavit]

On Wed, Oct 25, 2023 at 7:46 AM Jason Gunthorpe <jgg at nvidia.com> wrote:
>
> On Tue, Oct 24, 2023 at 02:34:28PM +0800, Michael Shavit wrote:
> > On Thu, Oct 12, 2023 at 7:26 AM Jason Gunthorpe <jgg at nvidia.com> wrote:
> > > [...]
> > > -static void arm_smmu_mmu_notifier_put(struct arm_smmu_mmu_notifier *smmu_mn)
> > > +static struct arm_smmu_ctx_desc *
> > > +arm_smmu_mmu_notifier_put(struct arm_smmu_mmu_notifier *smmu_mn)
> > >  {
> > >         struct mm_struct *mm = smmu_mn->mn.mm;
> > >         struct arm_smmu_ctx_desc *cd = smmu_mn->cd;
> > >         struct arm_smmu_domain *smmu_domain = smmu_mn->domain;
> > > -       struct arm_smmu_master *master;
> > > -       unsigned long flags;
> > >
> > >         if (!refcount_dec_and_test(&smmu_mn->refs))
> > > -               return;
> > > +               return cd;
> > >
> > >         list_del(&smmu_mn->list);
> > >
> > > -       spin_lock_irqsave(&smmu_domain->devices_lock, flags);
> > > -       list_for_each_entry(master, &smmu_domain->devices, domain_head)
> > > -               arm_smmu_clear_cd(master, mm->pasid);
> > > -       spin_unlock_irqrestore(&smmu_domain->devices_lock, flags);
> > > -
> > >         /*
> > >          * If we went through clear(), we've already invalidated, and no
> > >          * new TLB entry can have been formed.
> >
> > This re-orders the TLB invalidation before the CD entry is cleared.
> > Couldn't a misbehaving device form TLB entries in this time interval
> > that we'd want to avoid?
>
> Hum.. No for the 'inv_asid', but yes for the 'atc_inv_domain'

Just to confirm, why "No for the 'inv_asid'"? My best guess:
1. Transactions don't hit the TLB entries unless there's a valid CD
configured with that ASID
2. You're relying on those TLB entries being cleared elsewhere in the
code, when freeing/reclaiming the ASID from the domain.

But this also makes me curious why we bother with an ASID invalidation
in the first place if it's not required for correctness.