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

[Alex Williamson]

On Mon, 3 Apr 2023 15:50:54 -0700
Reinette Chatre <reinette.chatre@xxxxxxxxx> wrote:

> Hi Alex,
> 
> On 4/3/2023 1:22 PM, Alex Williamson wrote:
> > On Mon, 3 Apr 2023 10:31:23 -0700
> > Reinette Chatre <reinette.chatre@xxxxxxxxx> wrote:
> >   
> >> Hi Alex,
> >>
> >> On 3/31/2023 3:24 PM, Alex Williamson wrote:  
> >>> On Fri, 31 Mar 2023 10:49:16 -0700
> >>> Reinette Chatre <reinette.chatre@xxxxxxxxx> wrote:    
> >>>> 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:
> >>>>>>      
> >>
> >> ...
> >>  
> >>>>>>> +		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    
> >>
> >> Apologies, but could you please elaborate on where the asymmetry is? I am
> >> not able to see a flow in this solution where the ctx allocated by
> >> vfio_msi_enable() is freed if the trigger is unset.  
> > 
> > The user first calls SET_IRQS to enable MSI-X with some number of
> > vectors with (potentially) an eventfd for each vector.  The user later
> > calls SET_IRQS passing a -1 eventfd for one or more of the vectors with
> > an eventfd initialized in the prior step.  Given that we find the ctx,
> > the ctx has a trigger, and assuming dynamic allocation is supported, the
> > ctx is freed and vfio_msi_set_vector_signal() returns w/o allocating a
> > new ctx.  We've de-allocated both the irq and context initialized from
> > vfio_msi_enable().  
> 
> This is correct. The comment I responded to was in regards to an unset
> trigger. The flow you describe is when a trigger is set. Not that
> it changes your point though, which is that vfio_msi_set_vector_signal()
> frees memory allocated by vfio_msi_enable(). This is clear to me. This
> is intended behavior. My concern is/was with the error path where a function
> failing may not be expected to change state, you address that concern below.
> 
> >>> allocated where is unnecessarily complex, how about a policy that    
> >>
> >> I do not see this as tracking where allocations are made. Instead I
> >> see it as containing/compartmentalizing state changes with the goal of
> >> making the code easier to understand and maintain. Specifically, new_ctx
> >> is used so that if vfio_msi_set_vector_signal() fails, the state 
> >> before and after vfio_msi_set_vector_signal() will be the same.  
> > 
> > That's not really possible given how we teardown the existing ctx
> > before configuring the new one and unwind to disable contexts in
> > vfio_msi_set_block()  
> 
> Very unlikely indeed. I agree.
> 
> >> I do agree that it makes vfio_msi_set_vector_signal() more complex
> >> and I can remove new_ctx if you find that this is unnecessary after
> >> considering the motivations behind its use.   
> > 
> > If the goal is to allow the user to swap one eventfd for another, where
> > the result will always be the new eventfd on success or the old eventfd
> > on error, I don't see that this code does that, or that we've ever
> > attempted to make such a guarantee.  If the ioctl errors, I think the
> > eventfds are generally deconfigured.   We certainly have the unwind code
> > that we discussed earlier that deconfigures all the vectors previously
> > touched in the loop (which seems to be another path where we could
> > de-allocate from the set of initial ctxs).  
> 
> Thank you for your patience in hearing and addressing my concerns. I plan
> to remove new_ctx in the next version.
> 
> >>> devices supporting vdev->has_dyn_msix only ever have active contexts
> >>> allocated?  Thanks,    
> >>
> >> What do you see as an "active context"? A policy that is currently enforced
> >> is that an allocated context always has an allocated interrupt associated
> >> with it. I do not see how this could be expanded to also require an
> >> enabled interrupt because interrupt enabling requires a trigger that
> >> may not be available.  
> > 
> > A context is essentially meant to track a trigger, ie. an eventfd
> > provided by the user.  In the static case all the irqs are necessarily
> > pre-allocated, therefore we had no reason to consider a dynamic array
> > for the contexts.  However, a given context is really only "active" if
> > it has a trigger, otherwise it's just a placeholder.  When the
> > placeholder is filled by an eventfd, the pre-allocated irq is enabled.  
> 
> I see.
> 
> > 
> > This proposal seems to be a hybrid approach, pre-allocating some
> > initial set of irqs and contexts and expecting the differentiation to
> > occur only when new vectors are added, though we have some disagreement
> > about this per above.  Unfortunately I don't see an API to enable MSI-X
> > without some vectors, so some pre-allocation of irqs seems to be
> > required regardless.  
> 
> Right. pci_alloc_irq_vectors() or equivalent continues to be needed to
> enable MSI-X. Even so, it does seem possible (within vfio_msi_enable())
> to just allocate one vector using pci_alloc_irq_vectors()
> and then immediately free it using pci_msix_free_irq(). What do you think?

QEMU does something similar but I think it can really only be described
as a hack.  In this case I think we can work with them being allocated
since that's essentially the static path.

> If I understand correctly this can be done without allocating any context
> and leave MSI-X enabled without any interrupts allocated. This could be a
> way to accomplish the "active context" policy for dynamic allocation.
> This is not a policy that can be applied broadly to interrupt contexts though
> because MSI and non-dynamic MSI-X could still have contexts with allocated
> interrupts without eventfd.

I think we could come up with wrappers that handle all cases, for
example:

int vfio_pci_alloc_irq(struct vfio_pci_core_device *vdev,
		       unsigned int vector, int irq_type)
{
	struct pci_dev *pdev = vdev->pdev;
	struct msi_map map;
	int irq;

	if (irq_type == VFIO_PCI_INTX_IRQ_INDEX)
		return pdev->irq ?: -EINVAL;

	irq = pci_irq_vector(pdev, vector);
	if (irq > 0 || irq_type == VFIO_PCI_MSI_IRQ_INDEX ||
	    !vdev->has_dyn_msix)
		return irq;

	map = pci_msix_alloc_irq_at(pdev, vector, NULL);

	return map.index;
}

void vfio_pci_free_irq(struct vfio_pci_core_device *vdev,
		       unsigned in vector, int irq_type)
{
	struct msi_map map;
	int irq;

	if (irq_type != VFIO_PCI_INTX_MSIX_INDEX ||
	    !vdev->has_dyn_msix)
		return;

	irq = pci_irq_vector(pdev, vector);
	map = { .index = vector, .virq = irq };

	if (WARN_ON(irq < 0))
		return;

	pci_msix_free_irq(pdev, msix_map);
}

At that point, maybe we'd check whether it makes sense to embed the irq
alloc/free within the ctx alloc/free.
 
> > But if non-active contexts were only placeholders in the pre-dynamic
> > world and we now manage them via a dynamic array, why is there any
> > pre-allocation of contexts without knowing the nature of the eventfd to
> > fill it?  We could have more commonality between cases if contexts are
> > always dynamically allocated, which might simplify differentiation of
> > the has_dyn_msix cases largely to wrappers allocating and freeing irqs.
> > Thanks,  
> 
> Thank you very much for your guidance. I will digest this some more and 
> see how wrappers could be used. In the mean time while trying to think how
> to unify this code I do think there is an issue in this patch in that
> the get_cached_msi_msg()/pci_write_msi_msg()
> should not be in an else branch.
> 
> Specifically, I think it needs to be:
> 	if (msix) {
> 		if (irq == -EINVAL) {
> 			/* dynamically allocate interrupt */
> 		}
> 		get_cached_msi_msg(irq, &msg);
> 		pci_write_msi_msg(irq, &msg);
> 	}

Yes, that's looked wrong to me all along, I think that resolves it.
Thanks,

Alex