Re: [PATCH v9 7/8] xen/arm: enable dom0 to use PCI devices with pci-passthrough=no

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.03.2025 11:56, Mykyta Poturai wrote:
> On 17.03.25 17:07, Jan Beulich wrote:
>> On 14.03.2025 14:34, Mykyta Poturai wrote:
>>> --- a/xen/arch/arm/pci/pci.c
>>> +++ b/xen/arch/arm/pci/pci.c
>>> @@ -16,9 +16,18 @@
>>>   #include <xen/device_tree.h>
>>>   #include <xen/errno.h>
>>>   #include <xen/init.h>
>>> +#include <xen/iommu.h>
>>>   #include <xen/param.h>
>>>   #include <xen/pci.h>
>>>   
>>> +bool is_pci_passthrough_enabled(bool dom0)
>>> +{
>>> +    if ( dom0 )
>>> +        return pci_passthrough_enabled || iommu_enabled;
>>
>> As I think I said before - the function's name now no longer expresses
>> what it really checks. That (imo heavily) misleading at the use sites
>> of this function.
> 
> I am afraid I don't understand your concern. It still checks if PCI 
> passthrough is enabled. With just the change that ARM needs some extra 
> logic for Dom0 PCI to work properly.

Conceptually there's no such thing as "pass through" for Dom0. Hence the
name of the function itself isn't correctly reflecting what it's checking
for. iommu_enabled is a prereq for pass-through to be enabled, but it
doesn't imply that's necessarily the case.

> Maybe change the parameter name to 
> something like "for_pci_hwdom"?

That may help below, yes. But not here.

> >>> @@ -85,7 +94,7 @@ static int __init pci_init(void)
>>>        * Enable PCI passthrough when has been enabled explicitly
>>>        * (pci-passthrough=on).
>>>        */
>>> -    if ( !pci_passthrough_enabled )
>>> +    if ( !is_pci_passthrough_enabled(true) )
>>
>> There's no Dom0 in sight anywhere here, is there? How can you pass true
>> as argument for the "dom0" parameter?
> 
> This should be read as "is pci passthrough enabled for Dom0?" and if it 
> is we definitely need to do a PCI init.
> 
> I've also done some investigations on possible ways to remove the 
> Dom0/other domains distinction, but I'm afraid this is the most 
> reasonable way to make PCI functional on Dom0 without explicitly 
> enabling PCI passthrough.
> 
> SMMU is configured to trigger a fault on all transactions by default and 
> we can't statically map PCI devices to Dom0, so the only other way is to 
> put PCI in full passthrough mode, which I think is not safe enough.
> And we also can't drop this patch as it was directly requested by Julien 
> here [1].
> 
>>> --- 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(true) )
>>>               return -EOPNOTSUPP;
>>
>> Seeing the function's parameter name, how do you know it's Dom0 calling
>> here?
> 
> Is this a functional or naming concern? If it is about naming then can 
> it also be solved by renaming the parameter?

The renaming suggested above would resolve this, yes. Whether "for_pci_hwdom"
or anything alike is a good parameter name is a different question.

> Regarding functional issues, I have assumed that only hwdom can make 
> physdev operations, but after checking it, this assumption seems to be 
> correct on x86 but wrong on Arm.
> I expected there would be a check in do_arm_physdev_op() or somewhere 
> near it, similar to how it is done in x86, but there are none. I'm not 
> sure if it is intentional or by mistake, I think it needs some 
> clarification by Arm folks.

Hmm, looking at x86'es do_physdev_op() I fear I can't see such a check there
either. And indeed there are certain PHYSDEVOP_* which DomU-s may also make
use of.

Jan