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

[Alex Williamson]

On Fri, 31 Mar 2023 10:49:16 -0700
Reinette Chatre <reinette.chatre@xxxxxxxxx> wrote:

> Hi Alex,
> 
> On 3/30/2023 3:42 PM, Alex Williamson wrote:
> > On Thu, 30 Mar 2023 16:40:50 -0600
> > Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:
> >   
> >> On Tue, 28 Mar 2023 14:53:34 -0700
> >> Reinette Chatre <reinette.chatre@xxxxxxxxx> wrote:
> >>  
> 
> ...
> 
> >>> 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);
> >>> +}  
> > 
> > Also, the function below should use this rather than open coding the
> > same now.  Thanks,  
> 
> It should, yes. Thank you. Will do.
> 
> 
> >>>  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?  
> 
> It is not obvious to me if you mean this with vfio-pci-core probe,
> but it looks like a change to vfio_pci_core_enable() may be
> appropriate with a snippet like below:
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index a743b98ba29a..a474ce80a555 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -533,6 +533,8 @@ int vfio_pci_core_enable(struct vfio_pci_core_device *vdev)
>  	} else
>  		vdev->msix_bar = 0xFF;
>  
> +	vdev->has_dyn_msix = pci_msix_can_alloc_dyn(pdev);
> +
>  	if (!vfio_vga_disabled() && vfio_pci_is_vga(pdev))
>  		vdev->has_vga = true;
>  
> Please do note that I placed it outside of the earlier "if (msix_pos)" since
> pci_msix_can_alloc_dyn() has its own "if (!dev->msix_cap)". If you prefer
> to keep all the vdev->*msix* together I can move it into the if statement.

Sure, just for common grouping I'd probably put it within the existing
msix_cap branch.
 
> With vdev->has_dyn_msix available "allow_dyn_alloc" can be dropped as you
> stated.
> 
> >>  
> >>> +
> >>>  	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 };
> >>  
> 
> Sure. Will do.
> 
> >>> +			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);  
> 
> Will do.
> 
> >>> +			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.  
> 
> Thank you. Will do. To keep this patch focused I plan to separate
> this change into a new prep patch that will be placed earlier in
> this series.

Ok.

> >>  
> >>> +			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 };
> >>  
> 
> Will do.
> 
> >>> +		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,  
> 
> new_ctx is not required for correctness but instead is used to keep
> the code symmetric. 
> Specifically, if the user enables MSI-X without providing triggers and
> then later assign triggers then an error path without new_ctx would unwind
> more than done in this function, it would free the context that
> was allocated within vfio_msi_enable(). 

Seems like we already have that asymmetry, if a trigger is unset we'll
free the ctx allocated by vfio_msi_enable().  Tracking which are
allocated where is unnecessarily complex, how about a policy that
devices supporting vdev->has_dyn_msix only ever have active contexts
allocated?  Thanks,

Alex