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

[Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>]

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

>> +    return pci_passthrough_enabled;
>> +}
> 
> Personally I also view it as undesirable that the global
> pci_passthrough_enabled is evaluated twice in this function. Better
> might be e.g.
> 
> bool is_pci_passthrough_enabled(bool dom0)
> {
>      if ( pci_passthrough_enabled )
>          return true;
> 
>      return dom0 && iommu_enabled;
> }
> 
> Yet then I'm not a maintainer of this code.
> 

Agree, will change in the next version.

>> @@ -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/arch/x86/include/asm/pci.h
>> +++ b/xen/arch/x86/include/asm/pci.h
>> @@ -50,7 +50,7 @@ extern int pci_mmcfg_config_num;
>>   extern struct acpi_mcfg_allocation *pci_mmcfg_config;
>>   
>>   /* Unlike ARM, PCI passthrough is always enabled for x86. */
>> -static always_inline bool is_pci_passthrough_enabled(void)
>> +static always_inline bool is_pci_passthrough_enabled(__maybe_unused bool 
>> dom0)
> 
> Function parmeters don't need such annotation.
> 

Got it.

>> --- 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?
> 
> Jan

Is this a functional or naming concern? If it is about naming then can 
it also be solved by renaming the parameter?

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.

[1] 
https://patchew.org/Xen/20230607030220.22698-1-stewart.hildebrand@xxxxxxx/#2731b06d-4a54-f51c-2285-ea2cf1fac3ba@xxxxxxx
-- 
Mykyta