Re: [PATCH v12 07/17] iommu: Try to allocate blocking domain when probing device

Jason Gunthorpe
2022-08-30T13:30:07Z

On Tue, Aug 30, 2022 at 09:46:01AM +0800, Baolu Lu wrote:
> On 2022/8/30 01:27, Jason Gunthorpe wrote:
> > On Mon, Aug 29, 2022 at 11:40:24AM +0800, Baolu Lu wrote:
> > > On 2022/8/26 22:52, Jason Gunthorpe wrote:
> > > > On Fri, Aug 26, 2022 at 08:11:31PM +0800, Lu Baolu wrote:
> > > > > Allocate the blocking domain when probing devices if the driver supports
> > > > > blocking domain allocation. Otherwise, revert to the previous behavior,
> > > > > that is, use UNMANAGED domain instead when the blocking domain is needed.
> > > > >
> > > > > Signed-off-by: Lu Baolu<baolu.lu@linux.intel.com>
> > > > > Tested-by: Zhangfei Gao<zhangfei.gao@linaro.org>
> > > > > Tested-by: Tony Zhu<tony.zhu@intel.com>
> > > > > ---
> > > > > drivers/iommu/iommu.c | 29 +++++++++++++++++------------
> > > > > 1 file changed, 17 insertions(+), 12 deletions(-)
> > > > This seems like a lot of overhead to allocate these things for every
> > > > group?
> > > >
> > > > Why not add a simple refcount on the blocking domain instead and
> > > > allocate the domain on the pasid attach like we do for ownership?
> > >
> > > I am working towards implementing static instance of blocking domain for
> > > each IOMMU driver, and then, there's no much overhead to allocate it in
> > > the probing device path.
> >
> > Well, I thought about that and I don't think we can get
> > there in a short order.
>
> Yes. Fair enough.
>
> > Would rather you progress this series without
> > getting entangled in such a big adventure
>
> Agreed. I will drop this patch and add below code in the iommu
> interface:
>
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -3219,6 +3219,26 @@ int iommu_attach_device_pasid(struct iommu_domain
> *domain,
> return -ENODEV;
>
> mutex_lock(&group->mutex);
> +
> + /*
> + * The underlying IOMMU driver needs to support blocking domain
> + * allocation and the callback to block DMA transactions with a
> + * specific PASID.
> + */
> + if (!group->blocking_domain) {
> + group->blocking_domain = __iommu_domain_alloc(dev->bus,
> + IOMMU_DOMAIN_BLOCKED);
> + if (!group->blocking_domain) {
> + ret = -ENODEV;
> + goto out_unlock;
> + }
> + }
> +
> + if (!group->blocking_domain->ops->set_dev_pasid) {
> + ret = -EOPNOTSUPP;
> + goto out_unlock;
> + }
> +
> curr = xa_cmpxchg(&group->pasid_array, pasid, NULL, domain,
> GFP_KERNEL);
> if (curr) {
> ret = xa_err(curr) ? : -EBUSY;
>
> Currently both ARM SMMUv3 and VT-d drivers use static blocking domain.
> Hence I didn't use a refcount for blocking domain release here.

I don't think that works in the general case, you can't just destroy
what is in group->blocking_domain..

Maybe all of this is just the good reason to go to a simple
device->ops->remove_dev_pasid() callback and forget about blocking
domain here.

Jason