Re: [PATCH v13 01/14] vpci: use per-domain PCI lock to protect vpci structure

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.02.2024 10:01, Roger Pau Monné wrote:
> On Tue, Feb 13, 2024 at 09:44:58AM +0100, Jan Beulich wrote:
>> On 13.02.2024 09:35, Roger Pau Monné wrote:
>>> On Fri, Feb 02, 2024 at 04:33:05PM -0500, Stewart Hildebrand wrote:
>>>> --- a/xen/include/xen/sched.h
>>>> +++ b/xen/include/xen/sched.h
>>>> @@ -462,7 +462,8 @@ struct domain
>>>>  #ifdef CONFIG_HAS_PCI
>>>>      struct list_head pdev_list;
>>>>      /*
>>>> -     * pci_lock protects access to pdev_list.
>>>> +     * pci_lock protects access to pdev_list. pci_lock also protects 
>>>> pdev->vpci
>>>> +     * structure from being removed.
>>>>       *
>>>>       * Any user *reading* from pdev_list, or from devices stored in 
>>>> pdev_list,
>>>>       * should hold either pcidevs_lock() or pci_lock in read mode. 
>>>> Optionally,
>>>> @@ -628,6 +629,18 @@ struct domain
>>>>      unsigned int cdf;
>>>>  };
>>>>  
>>>> +/*
>>>> + * Check for use in ASSERTs to ensure that:
>>>> + *   1. we can *read* d->pdev_list
>>>> + *   2. pdevs (belonging to this domain) do not go away
>>>> + *   3. pdevs (belonging to this domain) do not get assigned to other 
>>>> domains
>>>
>>> I think you can just state that this check ensures there will be no
>>> changes to the entries in d->pdev_list, but not the contents of each
>>> entry.  No changes to d->pdev_list already ensures not devices can be
>>> deassigned or removed from the system, and obviously makes the list
>>> safe to iterate against.
>>>
>>> I would also drop the explicitly mention this is intended for ASSERT
>>> usage: there's nothing specific in the code that prevents it from
>>> being used in other places (albeit I think that's unlikely).
>>
>> But pcidevs_locked(), resolving to spin_is_locked(), isn't reliable. The
>> assertion usage is best-effort only, without a guarantee that all wrong
>> uses would be caught.
> 
> Do we want to protect this with !NDEBUG guards then?

Yes, that would look to be desirable.

>>>> + * This check is not suitable for protecting other state or critical 
>>>> regions.
>>>> + */
>>>> +#define pdev_list_is_read_locked(d) ({                           \
>>>
>>> I would be tempted to drop at least the '_read_' part from the name,
>>> the name is getting a bit too long for my taste.
>>
>> While I agree with the long-ish aspect, I'm afraid the "read" part is
>> crucial. As a result I see no room for shortening.
> 
> OK, if you think that's crucial then I'm not going to argue.
> 
>>>> +        struct domain *d_ = (d);                                 \
>>>
>>> Why do you need this local domain variable?  Can't you use the d
>>> parameter directly?
>>
>> It would be evaluated then somewhere between 0 and 2 times.
> 
> It's ASSERT code only, so I don't see that as an issue.

Fair point.

>  Otherwise d_ needs to be made const.

Indeed, but for assert-only code I agree the option is slightly better,
ideally suitably commented upon.

Jan