Re: [PATCH V2 7/8] vfio/pci: Support dynamic MSI-x

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 28 Mar 2023 14:53:34 -0700
Reinette Chatre <reinette.chatre@intel.com> wrote:

> Recently introduced pci_msix_alloc_irq_at() and pci_msix_free_irq()
> enables an individual MSI-X index to be allocated and freed after
> MSI-X enabling.
> 
> Support dynamic MSI-X if supported by the device. Keep the association
> between allocated interrupt and vfio interrupt context. Allocate new
> context together with the new interrupt if no interrupt context exist
> for an MSI-X interrupt. Similarly, release an interrupt with its
> context.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
> Changes since RFC V1:
> - Add pointer to interrupt context as function parameter to
>   vfio_irq_ctx_free(). (Alex)
> - Initialize new_ctx to false. (Dan Carpenter)
> - Only support dynamic allocation if device supports it. (Alex)
> 
>  drivers/vfio/pci/vfio_pci_intrs.c | 93 +++++++++++++++++++++++++------
>  1 file changed, 76 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> index b3a258e58625..755b752ca17e 100644
> --- a/drivers/vfio/pci/vfio_pci_intrs.c
> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> @@ -55,6 +55,13 @@ struct vfio_pci_irq_ctx *vfio_irq_ctx_get(struct vfio_pci_core_device *vdev,
>  	return xa_load(&vdev->ctx, index);
>  }
>  
> +static void vfio_irq_ctx_free(struct vfio_pci_core_device *vdev,
> +			      struct vfio_pci_irq_ctx *ctx, unsigned long index)
> +{
> +	xa_erase(&vdev->ctx, index);
> +	kfree(ctx);
> +}
> +
>  static void vfio_irq_ctx_free_all(struct vfio_pci_core_device *vdev)
>  {
>  	struct vfio_pci_irq_ctx *ctx;
> @@ -409,33 +416,62 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_core_device *vdev,
>  {
>  	struct pci_dev *pdev = vdev->pdev;
>  	struct vfio_pci_irq_ctx *ctx;
> +	struct msi_map msix_map = {};
> +	bool allow_dyn_alloc = false;
>  	struct eventfd_ctx *trigger;
> +	bool new_ctx = false;
>  	int irq, ret;
>  	u16 cmd;
>  
> +	/* Only MSI-X allows dynamic allocation. */
> +	if (msix && pci_msix_can_alloc_dyn(vdev->pdev))
> +		allow_dyn_alloc = true;

Should vfio-pci-core probe this and store it in a field on
vfio_pci_core_device so that we can simply use something like
vdev->has_dyn_msix throughout?

> +
>  	ctx = vfio_irq_ctx_get(vdev, vector);
> -	if (!ctx)
> +	if (!ctx && !allow_dyn_alloc)
>  		return -EINVAL;
> +
>  	irq = pci_irq_vector(pdev, vector);
> +	/* Context and interrupt are always allocated together. */
> +	WARN_ON((ctx && irq == -EINVAL) || (!ctx && irq != -EINVAL));
>  
> -	if (ctx->trigger) {
> +	if (ctx && ctx->trigger) {
>  		irq_bypass_unregister_producer(&ctx->producer);
>  
>  		cmd = vfio_pci_memory_lock_and_enable(vdev);
>  		free_irq(irq, ctx->trigger);
> +		if (allow_dyn_alloc) {

It almost seems easier to define msix_map in each scope that it's used:

			struct msi_map map = { .index = vector,
					       .virq = irq };

> +			msix_map.index = vector;
> +			msix_map.virq = irq;
> +			pci_msix_free_irq(pdev, msix_map);
> +			irq = -EINVAL;
> +		}
>  		vfio_pci_memory_unlock_and_restore(vdev, cmd);
>  		kfree(ctx->name);
>  		eventfd_ctx_put(ctx->trigger);
>  		ctx->trigger = NULL;
> +		if (allow_dyn_alloc) {
> +			vfio_irq_ctx_free(vdev, ctx, vector);
> +			ctx = NULL;
> +		}
>  	}
>  
>  	if (fd < 0)
>  		return 0;
>  
> +	if (!ctx) {
> +		ctx = vfio_irq_ctx_alloc_single(vdev, vector);
> +		if (!ctx)
> +			return -ENOMEM;
> +		new_ctx = true;
> +	}
> +
>  	ctx->name = kasprintf(GFP_KERNEL_ACCOUNT, "vfio-msi%s[%d](%s)",
>  			      msix ? "x" : "", vector, pci_name(pdev));
> -	if (!ctx->name)
> -		return -ENOMEM;
> +	if (!ctx->name) {
> +		ret = -ENOMEM;
> +		goto out_free_ctx;
> +	}
>  
>  	trigger = eventfd_ctx_fdget(fd);
>  	if (IS_ERR(trigger)) {
> @@ -443,25 +479,38 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_core_device *vdev,
>  		goto out_free_name;
>  	}
>  
> -	/*
> -	 * The MSIx vector table resides in device memory which may be cleared
> -	 * via backdoor resets. We don't allow direct access to the vector
> -	 * table so even if a userspace driver attempts to save/restore around
> -	 * such a reset it would be unsuccessful. To avoid this, restore the
> -	 * cached value of the message prior to enabling.
> -	 */
>  	cmd = vfio_pci_memory_lock_and_enable(vdev);
>  	if (msix) {
> -		struct msi_msg msg;
> -
> -		get_cached_msi_msg(irq, &msg);
> -		pci_write_msi_msg(irq, &msg);
> +		if (irq == -EINVAL) {
> +			msix_map = pci_msix_alloc_irq_at(pdev, vector, NULL);

			struct msi_map map = pci_msix_alloc_irq_at(pdev,
								vector, NULL);
> +			if (msix_map.index < 0) {
> +				vfio_pci_memory_unlock_and_restore(vdev, cmd);
> +				ret = msix_map.index;
> +				goto out_put_eventfd_ctx;
> +			}
> +			irq = msix_map.virq;
> +		} else {
> +			/*
> +			 * The MSIx vector table resides in device memory which
> +			 * may be cleared via backdoor resets. We don't allow
> +			 * direct access to the vector table so even if a
> +			 * userspace driver attempts to save/restore around
> +			 * such a reset it would be unsuccessful. To avoid
> +			 * this, restore the cached value of the message prior
> +			 * to enabling.
> +			 */

You've only just copied this comment down to here, but I think it's a
bit stale.  Maybe we should update it to something that helps explain
this split better, maybe:

			/*
			 * If the vector was previously allocated, refresh the
			 * on-device message data before enabling in case it had
			 * been cleared or corrupted since writing.
			 */

IIRC, that was the purpose of writing it back to the device and the
blocking of direct access is no longer accurate anyway.

> +			struct msi_msg msg;
> +
> +			get_cached_msi_msg(irq, &msg);
> +			pci_write_msi_msg(irq, &msg);
> +		}
>  	}
>  
>  	ret = request_irq(irq, vfio_msihandler, 0, ctx->name, trigger);
> -	vfio_pci_memory_unlock_and_restore(vdev, cmd);
>  	if (ret)
> -		goto out_put_eventfd_ctx;
> +		goto out_free_irq_locked;
> +
> +	vfio_pci_memory_unlock_and_restore(vdev, cmd);
>  
>  	ctx->producer.token = trigger;
>  	ctx->producer.irq = irq;
> @@ -477,11 +526,21 @@ static int vfio_msi_set_vector_signal(struct vfio_pci_core_device *vdev,
>  
>  	return 0;
>  
> +out_free_irq_locked:
> +	if (allow_dyn_alloc && new_ctx) {

		struct msi_map map = { .index = vector,
				       .virq = irq };

> +		msix_map.index = vector;
> +		msix_map.virq = irq;
> +		pci_msix_free_irq(pdev, msix_map);
> +	}
> +	vfio_pci_memory_unlock_and_restore(vdev, cmd);
>  out_put_eventfd_ctx:
>  	eventfd_ctx_put(trigger);
>  out_free_name:
>  	kfree(ctx->name);
>  	ctx->name = NULL;
> +out_free_ctx:
> +	if (allow_dyn_alloc && new_ctx)
> +		vfio_irq_ctx_free(vdev, ctx, vector);
>  	return ret;
>  }
>  

Do we really need the new_ctx test in the above cases?  Thanks,

Alex