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

[Reinette Chatre]

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.

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.

>>
>>> +			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(). 

Reinette