Re: [PATCH v5 01/15] s390/vfio-ap: Provide function to get the number of queues assigned to mdev

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Extremely inefficient nested bitmask iteration to count the number of queues.
--

--- Patch [1]: [PATCH v5 01/15] s390/vfio-ap: Provide function to get the number of queues assigned to mdev ---
commit c5c27f83fbf7c245f4954e58e37786067ff32658
Author: Anthony Krowiak <akrowiak@linux.ibm.com>
s390/vfio-ap: Provide function to get the number of queues assigned to mdev

Provides a function that returns the number of queues that are or will be
passed through to a guest when the mdev is attached to a guest.

> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> index 44b3a1dcc1b36..71d3e2c2e07c6 100644
> --- a/drivers/s390/crypto/vfio_ap_ops.c
> +++ b/drivers/s390/crypto/vfio_ap_ops.c

[ ... ]

> @@ -2098,6 +2098,20 @@ static void vfio_ap_mdev_request(struct vfio_device *vdev, unsigned int count)
>  	release_update_locks_for_mdev(matrix_mdev);
>  }
>  
> +int vfio_ap_mdev_get_num_queues(struct ap_matrix *ap_matrix)
> +{
> +	unsigned long apid, apqi;
> +	int num_queues = 0;
> +
> +	lockdep_assert_held(&matrix_dev->mdevs_lock);
> +
> +	for_each_set_bit_inv(apid, ap_matrix->apm, AP_DEVICES)
> +		for_each_set_bit_inv(apqi, ap_matrix->aqm, AP_DOMAINS)
> +			num_queues++;

[Severity: Medium]
Is this nested iteration the most efficient way to calculate the total number
of queues?

On s390, find_first_bit_inv and find_next_bit_inv are external function calls.
If a significant number of bits are set in both masks, this loop will make
thousands of function calls simply to count bits, resulting in an O(N*M)
operation instead of O(1).

Could we instead multiply the weights of the two bitmaps directly without loops?

return bitmap_weight(ap_matrix->apm, AP_DEVICES) *
       bitmap_weight(ap_matrix->aqm, AP_DOMAINS);

> +
> +	return num_queues;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260724161351.1802644-1-akrowiak@linux.ibm.com?part=1