On Thu, Sep 30, 2021 at 01:10:29PM +1000, David Gibson wrote:

On Wed, Sep 29, 2021 at 09:24:57AM -0300, Jason Gunthorpe wrote:
65;6402;1c> On Wed, Sep 29, 2021 at 03:25:54PM +1000, David Gibson wrote:

quoted

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+struct iommufd_device {
+	unsigned int id;
+	struct iommufd_ctx *ictx;
+	struct device *dev; /* always be the physical device */
+	u64 dev_cookie;

Why do you need both an 'id' and a 'dev_cookie'?  Since they're both
unique, couldn't you just use the cookie directly as the index into
the xarray?

ID is the kernel value in the xarray - xarray is much more efficient &
safe with small kernel controlled values.

dev_cookie is a user assigned value that may not be unique. It's
purpose is to allow userspace to receive and event and go back to its
structure. Most likely userspace will store a pointer here, but it is
also possible userspace could not use it.

It is a pretty normal pattern

Hm, ok.  Could you point me at an example?

For instance user_data vs fd in io_uring

RDMA has many similar examples.

More or less anytime you want to allow the kernel to async retun some
information providing a 64 bit user_data lets userspace have an easier
time to deal with it.

Jason