Re: [PATCH] virtio_pci: do not mutate caller irq_affinity.pre_vectors

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 05, 2026 at 11:29:34AM +0800, Xiong Weimin wrote:
> vp_request_msix_vectors() bumps desc->pre_vectors in place to reserve
> the virtio config vector. vp_find_vqs() may call it more than once while
> retrying MSI-X with different per-vq policies, so the caller's
> irq_affinity keeps accumulating and later attempts get the wrong
> affinity layout.
> 
> Copy the descriptor to a stack local, adjust pre_vectors there, and
> pass that to pci_alloc_irq_vectors_affinity().
> 
> Fixes: ba74b6f7fcc0 ("virtio_pci: fix cpu affinity support")
> Cc: stable@vger.kernel.org
> Signed-off-by: Xiong Weimin <xiongweimin@kylinos.cn>
> ---
>  drivers/virtio/virtio_pci_common.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> index 164f480b1..9e75fd03e 100644
> --- a/drivers/virtio/virtio_pci_common.c
> +++ b/drivers/virtio/virtio_pci_common.c
> @@ -128,6 +128,7 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
>  {
>  	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
>  	const char *name = dev_name(&vp_dev->vdev.dev);
> +	struct irq_affinity tmp_aff, *aff = NULL;


"tmp" in what sense? if we can come up with a sensible name, just
"affinity" will do.


>  	unsigned int flags = PCI_IRQ_MSIX;
>  	unsigned int i, v;
>  	int err = -ENOMEM;
> @@ -146,16 +147,19 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
>  					GFP_KERNEL))
>  			goto error;
>  
> -	if (!per_vq_vectors)
> -		desc = NULL;
> -
> -	if (desc) {
> +	if (per_vq_vectors && desc) {
> +		/*
> +		 * Do not mutate the caller's irq_affinity across MSI-X
> +		 * fallback retries in vp_find_vqs().
> +		 */


We do not need a comment arguing with previous version of the code.

> +		tmp_aff = *desc;
> +		tmp_aff.pre_vectors++; /* virtio config vector */
> +		aff = &tmp_aff;


just assign to desc, instead?

>  		flags |= PCI_IRQ_AFFINITY;
> -		desc->pre_vectors++; /* virtio config vector */
>  	}
>  
>  	err = pci_alloc_irq_vectors_affinity(vp_dev->pci_dev, nvectors,
> -					     nvectors, flags, desc);
> +					     nvectors, flags, aff);
>  	if (err < 0)
>  		goto error;
>  	vp_dev->msix_enabled = 1;
> -- 
> 2.43.0