Re: [PATCH V2 4/8] vfio/pci: Use xarray for interrupt context storage

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Jing,

On 4/7/2023 12:21 AM, Liu, Jing2 wrote:
> Hi Reinette,
> 
>> From: Chatre, Reinette <reinette.chatre@intel.com>
>> Subject: [PATCH V2 4/8] vfio/pci: Use xarray for interrupt context storage
>>
>> Interrupt context is statically allocated at the time interrupts are allocated.
>> Following allocation, the context is managed by directly accessing the
> elements of the array using the vector as index. The storage is release
> when interrupts are disabled.
>>
> 
> Looking at the dynamic allocation change for the time after MSI-x is
> enabled in vfio_msi_set_vector_signal(), do we need to consider changing 
> the allocation of context/interrupt in vfio_msi_enable() for MSI-x to the 
> same way, which only allocates for vectors with a valid signal fd when 
> dynamic MSI-x is supported?
> 
> Because in dynamic logic, I think when enabling MSI-x, not all vectors from 
> zero to nvec should be allocated, and they would be done until they are really
> used with setting the singal fd.
> 
> Not sure if this comment being replied here is the good place since
> vfio_msi_enable() seems no change in this series. 😊 

This is a good question and from what I understand it could be done either way.

vfio_msi_enable()->pci_alloc_irq_vectors() would always be required because
pci_alloc_irq_vectors() enables MSI-X in addition to allocating the vectors.

I expect it to be efficient to allocate a range using pci_alloc_irq_vectors()
at the same time as MSI-X enabling in anticipation of their subsequent activation
after needing to only take the vfio and MSI locks once.

As you indicate, it is also possible to only allocate one vector during MSI-X
enabling using pci_alloc_irq_vectors(), leaving the other allocations to
vfio_msi_set_vector_signal(). Not a major issue but this would require some
additional special cases within vfio_msi_enable() while the current solution
allocating all vectors using pci_alloc_irq_vectors() works for all types.

Considering which would be more efficient would depend on the use cases. The
current solution may be considered less efficient if the user enables MSI-X
by providing a range of vectors without triggers(*). From what I can tell
this is not a possible use case when using Qemu. Qemu enables MSI-X by
calling VFIO_DEVICE_SET_IRQS for vector 0 with a trigger. Making a change
to only allocate vector 0 using pci_alloc_irq_vectors() and the rest using
vfio_msi_set_vector_signal() would thus have no impact on Qemu. Both
implementations behave the same for Qemu.

Considering the efficiency of allocating multiple vectors together that
works for all interrupts (MSI, non dynamic MSI-X, and dynamic MSI-X) without
any impact to Qemu I do lean towards keeping the current implementation.

Reinette

(*) Whether it is less efficient may possibly be debated considering that 
it may be desirable to use allocated interrupts as a cache:
https://lore.kernel.org/lkml/20230404122444.59e36a99.alex.williamson@redhat.com/