Re: [PATCH v4 02/26] iommu/sva: Manage process address spaces

Jonathan Cameron
2020-02-28T16:26:44Z

On Fri, 28 Feb 2020 15:43:04 +0100
Jean-Philippe Brucker <jean-philippe@linaro.org> wrote:

> On Wed, Feb 26, 2020 at 12:35:06PM +0000, Jonathan Cameron wrote:
> > > + * A single Process Address Space ID (PASID) is allocated for each mm. In the
> > > + * example, devices use PASID 1 to read/write into address space X and PASID 2
> > > + * to read/write into address space Y. Calling iommu_sva_get_pasid() on bond 1
> > > + * returns 1, and calling it on bonds 2-4 returns 2.
> > > + *
> > > + * Hardware tables describing this configuration in the IOMMU would typically
> > > + * look like this:
> > > + *
> > > + * PASID tables
> > > + * of domain A
> > > + * .->+--------+
> > > + * / 0 | |-------> io_pgtable
> > > + * / +--------+
> > > + * Device tables / 1 | |-------> pgd X
> > > + * +--------+ / +--------+
> > > + * 00:00.0 | A |-' 2 | |--.
> > > + * +--------+ +--------+ \
> > > + * : : 3 | | \
> > > + * +--------+ +--------+ --> pgd Y
> > > + * 00:01.0 | B |--. /
> > > + * +--------+ \ |
> > > + * 00:01.1 | B |----+ PASID tables |
> > > + * +--------+ \ of domain B |
> > > + * '->+--------+ |
> > > + * 0 | |-- | --> io_pgtable
> > > + * +--------+ |
> > > + * 1 | | |
> > > + * +--------+ |
> > > + * 2 | |---'
> > > + * +--------+
> > > + * 3 | |
> > > + * +--------+
> > > + *
> > > + * With this model, a single call binds all devices in a given domain to an
> > > + * address space. Other devices in the domain will get the same bond implicitly.
> > > + * However, users must issue one bind() for each device, because IOMMUs may
> > > + * implement SVA differently. Furthermore, mandating one bind() per device
> > > + * allows the driver to perform sanity-checks on device capabilities.
> >
> > > + *
> > > + * In some IOMMUs, one entry of the PASID table (typically the first one) can
> > > + * hold non-PASID translations. In this case PASID 0 is reserved and the first
> > > + * entry points to the io_pgtable pointer. In other IOMMUs the io_pgtable
> > > + * pointer is held in the device table and PASID 0 is available to the
> > > + * allocator.
> >
> > Is it worth hammering home in here that we can only do this because the PASID space
> > is global (with exception of PASID 0)? It's a convenient simplification but not
> > necessarily a hardware restriction so perhaps we should remind people somewhere in here?
>
> I could add this four paragraphs up:
>
> "A single Process Address Space ID (PASID) is allocated for each mm. It is
> a choice made for the Linux SVA implementation, not a hardware
> restriction."

Perfect.

>
> > > + */
> > > +
> > > +struct io_mm {
> > > + struct list_head devices;
> > > + struct mm_struct *mm;
> > > + struct mmu_notifier notifier;
> > > +
> > > + /* Late initialization */
> > > + const struct io_mm_ops *ops;
> > > + void *ctx;
> > > + int pasid;
> > > +};
> > > +
> > > +#define to_io_mm(mmu_notifier) container_of(mmu_notifier, struct io_mm, notifier)
> > > +#define to_iommu_bond(handle) container_of(handle, struct iommu_bond, sva)
> >
> > Code ordering wise, do we want this after the definition of iommu_bond?
> >
> > For both of these it's a bit non obvious what they come 'from'.
> > I wouldn't naturally assume to_io_mm gets me from notifier to the io_mm
> > for example. Not sure it matters though if these are only used in a few
> > places.
>
> Right, I can rename the first one to mn_to_io_mm(). The second one I think
> might be good enough.

Agreed. The second one does feel more natural.

>
>
> > > +static struct iommu_sva *
> > > +io_mm_attach(struct device *dev, struct io_mm *io_mm, void *drvdata)
> > > +{
> > > + int ret = 0;
> >
> > I'm fairly sure this is set in all paths below. Now, of course the
> > compiler might not think that in which case fair enough :)
> >
> > > + bool attach_domain = true;
> > > + struct iommu_bond *bond, *tmp;
> > > + struct iommu_domain *domain, *other;
> > > + struct iommu_sva_param *param = dev->iommu_param->sva_param;
> > > +
> > > + domain = iommu_get_domain_for_dev(dev);
> > > +
> > > + bond = kzalloc(sizeof(*bond), GFP_KERNEL);
> > > + if (!bond)
> > > + return ERR_PTR(-ENOMEM);
> > > +
> > > + bond->sva.dev = dev;
> > > + bond->drvdata = drvdata;
> > > + refcount_set(&bond->refs, 1);
> > > + RCU_INIT_POINTER(bond->io_mm, io_mm);
> > > +
> > > + mutex_lock(&iommu_sva_lock);
> > > + /* Is it already bound to the device or domain? */
> > > + list_for_each_entry(tmp, &io_mm->devices, mm_head) {
> > > + if (tmp->sva.dev != dev) {
> > > + other = iommu_get_domain_for_dev(tmp->sva.dev);
> > > + if (domain == other)
> > > + attach_domain = false;
> > > +
> > > + continue;
> > > + }
> > > +
> > > + if (WARN_ON(tmp->drvdata != drvdata)) {
> > > + ret = -EINVAL;
> > > + goto err_free;
> > > + }
> > > +
> > > + /*
> > > + * Hold a single io_mm reference per bond. Note that we can't
> > > + * return an error after this, otherwise the caller would drop
> > > + * an additional reference to the io_mm.
> > > + */
> > > + refcount_inc(&tmp->refs);
> > > + io_mm_put(io_mm);
> > > + kfree(bond);
> >
> > Free outside the lock would be ever so slightly more logical given we allocated
> > before taking the lock.
> >
> > > + mutex_unlock(&iommu_sva_lock);
> > > + return &tmp->sva;
> > > + }
> > > +
> > > + list_add_rcu(&bond->mm_head, &io_mm->devices);
> > > + param->nr_bonds++;
> > > + mutex_unlock(&iommu_sva_lock);
> > > +
> > > + ret = io_mm->ops->attach(bond->sva.dev, io_mm->pasid, io_mm->ctx,
> > > + attach_domain);
> > > + if (ret)
> > > + goto err_remove;
> > > +
> > > + return &bond->sva;
> > > +
> > > +err_remove:
> > > + /*
> > > + * At this point concurrent threads may have started to access the
> > > + * io_mm->devices list in order to invalidate address ranges, which
> > > + * requires to free the bond via kfree_rcu()
> > > + */
> > > + mutex_lock(&iommu_sva_lock);
> > > + param->nr_bonds--;
> > > + list_del_rcu(&bond->mm_head);
> > > +
> > > +err_free:
> > > + mutex_unlock(&iommu_sva_lock);
> > > + kfree_rcu(bond, rcu_head);
> >
> > I don't suppose it matters really but we don't need the rcu free if
> > we follow the err_free goto. Perhaps we are cleaner in this case
> > to not use a unified exit path but do that case inline?
>
> Agreed, though I moved the kzalloc() later as suggested by Jacob, I think
> it looks a little better and simplifies the error paths
>
> Thanks,
> Jean
Jonathan