Re: [PATCH v2 11/11] xen/arm: Translate virtual PCI bus topology for guests

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.09.2021 15:43, Oleksandr Andrushchenko wrote:
> 
> On 27.09.21 16:34, Jan Beulich wrote:
>> On 27.09.2021 14:08, Oleksandr Andrushchenko wrote:
>>> On 27.09.21 14:31, Jan Beulich wrote:
>>>> On 23.09.2021 14:55, Oleksandr Andrushchenko wrote:
>>>>> --- a/xen/drivers/passthrough/pci.c
>>>>> +++ b/xen/drivers/passthrough/pci.c
>>>>> @@ -890,6 +890,31 @@ int pci_remove_virtual_device(struct domain *d, 
>>>>> const struct pci_dev *pdev)
>>>>>        return 0;
>>>>>    }
>>>>>    
>>>>> +/*
>>>>> + * Find the physical device which is mapped to the virtual device
>>>>> + * and translate virtual SBDF to the physical one.
>>>>> + */
>>>>> +bool pci_translate_virtual_device(struct vcpu *v, pci_sbdf_t *sbdf)
>>>> Why struct vcpu, when you only need ...
>>>>
>>>>> +{
>>>>> +    struct domain *d = v->domain;
>>>> ... this? It's also not really logical for this function to take a
>>>> struct vcpu, as the translation should be uniform within a domain.
>>> Agree, struct domain is just enough
>>>> Also - const please (as said elsewhere before, ideally wherever possible
>>>> and sensible).
>>> Ok
>>>>> +    struct vpci_dev *vdev;
>>>>> +    bool found = false;
>>>>> +
>>>>> +    pcidevs_lock();
>>>>> +    list_for_each_entry ( vdev, &d->vdev_list, list )
>>>>> +    {
>>>>> +        if ( vdev->sbdf.sbdf == sbdf->sbdf )
>>>>> +        {
>>>>> +            /* Replace virtual SBDF with the physical one. */
>>>>> +            *sbdf = vdev->pdev->sbdf;
>>>>> +            found = true;
>>>>> +            break;
>>>>> +        }
>>>>> +    }
>>>> For a DomU with just one or at most a couple of devices, such a brute
>>>> force lookup may be fine. What about Dom0 though? The physical topology
>>>> gets split at the segment level, so maybe this would by a reasonable
>>>> granularity here as well?
>>> Not sure I am following why topology matters here. We are just trying to
>>> match one SBDF (as seen by the guest) to other SBDF (physical,
>>> as seen by Dom0), so we can proxy DomU's configuration space access
>>> to the proper device in Dom0.
>> Topology here matters only in so far as I've suggested to have separate
>> lists per segment, to reduce look times. Other methods of avoiding a
>> fully linear search are of course possible as well.
> 
> Ah, with that that respect then of course. But let's be realistic.
> How many PCI devices are normally passed through to a guest?
> I can assume this is probably less than 10 most of the time.
> By assuming that the number of devices is small I see no profit,
> but unneeded complexity in accounting virtual devices per segment
> and performing the relevant lookup. So, I would go with a single list
> and "brute force lookup" unless it is clearly seen that this needs to be
> optimized.


Just to repeat my initial reply: "For a DomU with just one or at most
a couple of devices, such a brute force lookup may be fine. What about
Dom0 though?" If the code uses the simpler form because it's only
going to be used for DomU, then that's fine for now. But such latent
issues will want recording - e.g. by TODO comments or at the very
least suitable pointing out in the description.

Jan