RE: [PATCH 1/8] iommu: Introduce a replace API for device pasid

[Tian, Kevin]

> From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> Sent: Sunday, March 10, 2024 9:06 PM
> 
> On 2024/1/16 01:19, Jason Gunthorpe wrote:
> > On Sun, Nov 26, 2023 at 10:34:21PM -0800, Yi Liu wrote:
> >> +int iommu_replace_device_pasid(struct iommu_domain *domain,
> >> +			       struct device *dev, ioasid_t pasid)
> >> +{
> >> +	struct iommu_group *group = dev->iommu_group;
> >> +	struct iommu_domain *old_domain;
> >> +	int ret;
> >> +
> >> +	if (!domain)
> >> +		return -EINVAL;
> >> +
> >> +	if (!group)
> >> +		return -ENODEV;
> >> +
> >> +	mutex_lock(&group->mutex);
> >> +	__iommu_remove_group_pasid(group, pasid);
> >
> > It is not replace if you do remove first.
> >
> > Replace must just call set_dev_pasid and nothing much else..
> 
> Seems uneasy to do it so far. The VT-d driver needs to get the old domain
> first in order to do replacement. However, VT-d driver does not track the
> attached domains of pasids. It gets domain of a pasid
> by iommu_get_domain_for_dev_pasid(). Like
> intel_iommu_remove_dev_pasid)
> in link [1]. While the iommu layer exchanges the domain in the
> group->pasid_array before calling into VT-d driver. So when calling into
> VT-d driver, the domain got by iommu_get_domain_for_dev_pasid() is
> already
> the new domain. To solve it, we may need to let VT-d driver have its
> own tracking on the domains. How about your thoughts? @Jason, @Kevin,
> @Baoplu?
> 
> [1]
> https://github.com/torvalds/linux/blob/master/drivers/iommu/intel/iommu
> .c#L4621C19-L4621C20
> 

Jason's point was that the core helper should directly call set_dev_pasid
and underlying iommu driver decides whether atomic replacement
can be implemented inside the callback. If you first remove in the core
then one can never implement a replace semantics.