Re: [PATCH v6 2/3] xen/arm: Enable the existing x86 virtual PCI support for ARM.

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.10.2021 11:52, Bertrand Marquis wrote:
>> On 15 Oct 2021, at 09:32, Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>> On Thu, Oct 14, 2021 at 03:49:50PM +0100, Bertrand Marquis wrote:
>>> @@ -752,6 +752,19 @@ int pci_add_device(u16 seg, u8 bus, u8 devfn,
>>>
>>>     check_pdev(pdev);
>>>
>>> +#ifdef CONFIG_ARM
>>> +    /*
>>> +     * On ARM PCI devices discovery will be done by Dom0. Add vpci handler 
>>> when
>>> +     * Dom0 inform XEN to add the PCI devices in XEN.
>>> +     */
>>> +    ret = vpci_add_handlers(pdev);
>>> +    if ( ret )
>>> +    {
>>> +        printk(XENLOG_ERR "Setup of vPCI failed: %d\n", ret);
>>> +        goto out;
>>> +    }
>>> +#endif
>>
>> I think vpci_add_handlers should be called after checking that
>> pdev->domain is != NULL, so I would move this chunk a bit below.
> 
> On arm this would prevent the dom0less use case or to have the PCI
> bus enumerated from an other domain.
> @oleksandr: can you comment on this one, you might have a better
> answer than me on this ?

Well, without Xen doing the enumeration, some other entity would need
to do so, including the reporting to Xen. Obviously without a Dom0 it
would be ambiguous which domain to assign the device to; perhaps it
should be the caller in this case? That would make that caller domain
a pseudo-hwdom though, as far as PCI is concerned, which may not be
desirable according to my (limited) understanding of dom0less.

>>> @@ -784,6 +797,9 @@ out:
>>>                    &PCI_SBDF(seg, bus, slot, func));
>>>         }
>>>     }
>>> +    else if ( pdev )
>>> +        pci_cleanup_msi(pdev);
>>
>> I'm slightly lost at why you add this chunk, is this strictly related
>> to the patch?
> 
> This was discussed a lot in previous version of the patch and
> requested by Stefano. The idea here is that as soon as handlers
> are added some bits might be modified in the PCI config space
> leading possibly to msi interrupts. So it is safer to cleanup on the
> error path. For references please see discussion on v4 and v5 where
> this was actually added (to much references as the discussion was
> long so here [1] and [2] are the patchwork thread).
> 
> [1] 
> https://patchwork.kernel.org/project/xen-devel/patch/9bdca2cda5d2e83f94dc2423e55714273539760a.1633540842.git.rahul.singh@xxxxxxx/
> [2] 
> https://patchwork.kernel.org/project/xen-devel/patch/f093de681c2560a7196895bcd666ef8840885c1d.1633340795.git.rahul.singh@xxxxxxx/

The addition of this call has repeatedly raised questions. This is a
good indication that sufficient discussion thereof has been lacking
from the patch description.

Jan