Re: [RFC PATCH 8/8] vfio/pci: Clear VFIO_IRQ_INFO_NORESIZE for MSI-X

[Reinette Chatre]

Hi Alex,

On 3/17/2023 4:01 PM, Alex Williamson wrote:
> On Fri, 17 Mar 2023 15:21:09 -0700
> Reinette Chatre <reinette.chatre@xxxxxxxxx> wrote:
> 
>> Hi Alex,
>>
>> On 3/17/2023 2:05 PM, Alex Williamson wrote:
>>> On Wed, 15 Mar 2023 13:59:28 -0700
>>> Reinette Chatre <reinette.chatre@xxxxxxxxx> wrote:
>>>   
>>>> Dynamic MSI-X is supported. Clear VFIO_IRQ_INFO_NORESIZE
>>>> to provide guidance to user space.
>>>>
>>>> Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
>>>> ---
>>>>  drivers/vfio/pci/vfio_pci_core.c | 2 +-
>>>>  include/uapi/linux/vfio.h        | 3 +++
>>>>  2 files changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
>>>> index ae0e161c7fc9..1d071ee212a7 100644
>>>> --- a/drivers/vfio/pci/vfio_pci_core.c
>>>> +++ b/drivers/vfio/pci/vfio_pci_core.c
>>>> @@ -1111,7 +1111,7 @@ static int vfio_pci_ioctl_get_irq_info(struct vfio_pci_core_device *vdev,
>>>>  	if (info.index == VFIO_PCI_INTX_IRQ_INDEX)
>>>>  		info.flags |=
>>>>  			(VFIO_IRQ_INFO_MASKABLE | VFIO_IRQ_INFO_AUTOMASKED);
>>>> -	else
>>>> +	else if (info.index != VFIO_PCI_MSIX_IRQ_INDEX)
>>>>  		info.flags |= VFIO_IRQ_INFO_NORESIZE;
>>>>    
>>>
>>> I think we need to check pci_msix_can_alloc_dyn(), right?  Thanks,  
>>
>> Can pci_msix_can_alloc_dyn() ever return false?
>>
>> I cannot see how pci_msix_can_alloc_dyn() can return false when
>> considering the VFIO PCI MSI-X flow:
>>
>> vfio_msi_enable(..., ..., msix == true) 
>>   pci_alloc_irq_vectors(..., ..., ..., flag == PCI_IRQ_MSIX) 
>>     pci_alloc_irq_vectors_affinity() 
>>       __pci_enable_msix_range() 
>>         pci_setup_msix_device_domain() 
>>           pci_create_device_domain(..., &pci_msix_template, ...)
>>
>> The template used above, pci_msix_template, has MSI_FLAG_PCI_MSIX_ALLOC_DYN
>> hardcoded. This is the flag that pci_msix_can_alloc_dyn() tests for.
>>
>> If indeed pci_msix_can_alloc_dyn() can return false in the VFIO PCI MSI-X
>> usage then this series needs to be reworked to continue supporting
>> existing allocation mechanism as well as dynamic MSI-X allocation. Which
>> allocation mechanism to use would be depend on pci_msix_can_alloc_dyn().
>>
>> Alternatively, if you agree that VFIO PCI MSI-X can expect
>> pci_msix_can_alloc_dyn() to always return true then I should perhaps
>> add a test in vfio_msi_enable() that confirms this is the case. This would
>> cause vfio_msi_enable() to fail if dynamic MSI-X is not possible, perhaps even
>> complain loudly with a WARN.
> 
> pci_setup_msix_device_domain() says it returns true if:
> 
>  *  True when:
>  *      - The device does not have a MSI parent irq domain associated,
>  *        which keeps the legacy architecture specific and the global
>  *        PCI/MSI domain models working
>  *      - The MSI-X domain exists already
>  *      - The MSI-X domain was successfully allocated
> 
> That first one seems concerning, dynamic allocation only works on irq
> domain configurations.  What does that exclude and do we care about any
> of them for vfio-pci?  Minimally this suggests a dependency on
> IRQ_DOMAIN, which we don't currently have, but I'm not sure if
> supporting irq domains is the same as having irq domains.  Thanks,

Just to confirm, as I mentioned in [1] I do plan to rework this solution
to support both allocation mechanisms, using pci_msix_can_alloc_dyn()
to pick which one to use. Thank you very much for pointing out this
gap to me.

Reinette

[1] https://lore.kernel.org/lkml/e2d3f5a6-0a36-f19d-8f19-748197c3308d@xxxxxxxxx/