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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

>> + * 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.

>> +        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.

Jan