Re: [PATCH v8 7/8] xen/arm: enable dom0 to use PCI devices with pci-passthrough=no

[Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>]

On 10.02.25 12:56, Jan Beulich wrote:
> On 10.02.2025 11:30, Mykyta Poturai wrote:
>> From: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
>>
>> Enable the use of IOMMU + PCI in dom0 without having to specify
>> "pci-passthrough=yes".
> 
> Why? It _is_ passing through, even if Dom0 is special.

Do you think it would be better to drop this completely and require
pci-passthrough=yes for PCI to work in Dom0?

> 
>> @@ -83,9 +84,9 @@ static int __init pci_init(void)
>>   {
>>       /*
>>        * Enable PCI passthrough when has been enabled explicitly
>> -     * (pci-passthrough=on).
>> +     * (pci-passthrough=on) or IOMMU is present and enabled.
>>        */
>> -    if ( !pci_passthrough_enabled )
>> +    if ( !is_pci_passthrough_enabled() && !iommu_enabled )
>>           return 0;
> 
> I can't reasonably judge on this adjustment, but ...
> 
>> --- a/xen/drivers/pci/physdev.c
>> +++ b/xen/drivers/pci/physdev.c
>> @@ -19,7 +19,7 @@ ret_t pci_physdev_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) 
>> arg)
>>           struct pci_dev_info pdev_info;
>>           nodeid_t node = NUMA_NO_NODE;
>>   
>> -        if ( !is_pci_passthrough_enabled() )
>> +        if ( !is_pci_passthrough_enabled() && !iommu_enabled )
>>               return -EOPNOTSUPP;
>>   
>>           ret = -EFAULT;
>> @@ -57,7 +57,7 @@ ret_t pci_physdev_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) 
>> arg)
>>       case PHYSDEVOP_pci_device_remove: {
>>           struct physdev_pci_device dev;
>>   
>> -        if ( !is_pci_passthrough_enabled() )
>> +        if ( !is_pci_passthrough_enabled() && !iommu_enabled )
>>               return -EOPNOTSUPP;
>>   
>>           ret = -EFAULT;
> 
> ... these two certainly look wrong to be made. If an Arm-specific adjustment
> is needed (and can be justified), a per-arch hook may need introducing.

This should not affect x86 in any way if I'm not missing something, as
!is_pci_passthrough_enabled() will always be false. Or are you concerned 
about something else?

> 
> Jan

-- 
Mykyta