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

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

On 2/13/24 04:05, Jan Beulich wrote:
> 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.

We will then also need a definition of pdev_list_is_read_locked() in the
#else case so we don't risk running into "error: implicit declaration of
function 'pdev_list_is_read_locked'".

Such a definition might look like:

#define pdev_list_is_read_locked(d) ({ (void)d; ASSERT_UNREACHABLE(); false; })

so that we still evaluate d exactly once in the NDEBUG case.

>>>>> + * 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.

Is "the option" here referring to making d_ const, or using d directly
(with suitable comment)?