Re: [PATCH v2 4/6] iommufd: Deliver fault messages to user space

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2023/12/1 23:24, Jason Gunthorpe wrote:
> On Thu, Oct 26, 2023 at 10:49:28AM +0800, Lu Baolu wrote:
> 
>> +static ssize_t hwpt_fault_fops_write(struct file *filep,
>> +				     const char __user *buf,
>> +				     size_t count, loff_t *ppos)
>> +{
>> +	size_t response_size = sizeof(struct iommu_hwpt_page_response);
>> +	struct hw_pgtable_fault *fault = filep->private_data;
>> +	struct iommu_hwpt_page_response response;
>> +	struct iommufd_hw_pagetable *hwpt;
>> +	struct iopf_group *iter, *group;
>> +	struct iommufd_device *idev;
>> +	size_t done = 0;
>> +	int rc = 0;
>> +
>> +	if (*ppos || count % response_size)
>> +		return -ESPIPE;
>> +
>> +	mutex_lock(&fault->mutex);
>> +	while (!list_empty(&fault->response) && count > done) {
>> +		rc = copy_from_user(&response, buf + done, response_size);
>> +		if (rc)
>> +			break;
>> +
>> +		/* Get the device that this response targets at. */
>> +		idev = container_of(iommufd_get_object(fault->ictx,
>> +						       response.dev_id,
>> +						       IOMMUFD_OBJ_DEVICE),
>> +				    struct iommufd_device, obj);
>> +		if (IS_ERR(idev)) {
>> +			rc = PTR_ERR(idev);
>> +			break;
>> +		}
> 
> See here it might be better to have a per-fd list of outstanding
> faults per-fd and then the cookie would just index that list, then you
> get everything in one shot instead of having to do a xarray looking
> and then a linear list search

Yours is more efficient. I will do it that way in the next version.

> 
>> +static const struct file_operations hwpt_fault_fops = {
>> +	.owner		= THIS_MODULE,
>> +	.read		= hwpt_fault_fops_read,
>> +	.write		= hwpt_fault_fops_write,
>> +};
> 
> nonseekable_open() behavior should be integrated into this

Sure.

> 
>> +static int hw_pagetable_get_fault_fd(struct hw_pgtable_fault *fault)
>> +{
>> +	struct file *filep;
>> +	int fdno;
>> +
>> +	fdno = get_unused_fd_flags(O_CLOEXEC);
>> +	if (fdno < 0)
>> +		return fdno;
>> +
>> +	filep = anon_inode_getfile("[iommufd-pgfault]", &hwpt_fault_fops,
>> +				   fault, O_RDWR);
>> +	if (IS_ERR(filep)) {
>> +		put_unused_fd(fdno);
>> +		return PTR_ERR(filep);
>> +	}
>> +
>> +	fd_install(fdno, filep);
>> +	fault->fault_file = filep;
>> +	fault->fault_fd = fdno;
> 
> fd_install must be the very last thing before returning success from a
> system call because we cannot undo it.

Yes.

> 
> There are other failure paths before here and the final return
> 
> Jason

Best regards,
baolu