Re: [PATCH 7/9] xen/x86: rename cache_flush_permitted() to has_arch_io_resources()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.05.2025 10:27, Roger Pau Monné wrote:
> On Fri, May 16, 2025 at 10:08:35AM +0200, Jan Beulich wrote:
>> On 16.05.2025 10:02, Roger Pau Monné wrote:
>>> On Fri, May 16, 2025 at 09:07:43AM +0200, Jan Beulich wrote:
>>>> On 15.05.2025 12:28, Roger Pau Monné wrote:
>>>>> On Mon, May 12, 2025 at 05:16:02PM +0200, Jan Beulich wrote:
>>>>>> On 06.05.2025 10:31, Roger Pau Monne wrote:
>>>>>>> To better describe the underlying implementation.  Define
>>>>>>> cache_flush_permitted() as an alias of has_arch_io_resources(), so that
>>>>>>> current users of cache_flush_permitted() are not effectively modified.
>>>>>>>
>>>>>>> With the introduction of the new handler, change some of the call sites 
>>>>>>> of
>>>>>>> cache_flush_permitted() to instead use has_arch_io_resources() as such
>>>>>>> callers are not after whether cache flush is enabled, but rather whether
>>>>>>> the domain has any IO resources assigned.
>>>>>>>
>>>>>>> Take the opportunity to adjust l1_disallow_mask() to use the newly
>>>>>>> introduced has_arch_io_resources() macro.
>>>>>>
>>>>>> While I'm happy with everything else here, to me it's at least on the
>>>>>> edge whether cache_flush_permitted() wouldn't be the better predicate
>>>>>> to use there, for this being about ...
>>>>>>
>>>>>>> --- a/xen/arch/x86/mm.c
>>>>>>> +++ b/xen/arch/x86/mm.c
>>>>>>> @@ -172,8 +172,7 @@ static DEFINE_SPINLOCK(subpage_ro_lock);
>>>>>>>  
>>>>>>>  #define l1_disallow_mask(d)                                     \
>>>>>>>      (((d) != dom_io) &&                                         \
>>>>>>> -     (rangeset_is_empty((d)->iomem_caps) &&                     \
>>>>>>> -      rangeset_is_empty((d)->arch.ioport_caps) &&               \
>>>>>>> +     (!has_arch_io_resources(d) &&                              \
>>>>>>>        !has_arch_pdevs(d) &&                                     \
>>>>>>>        is_pv_domain(d)) ?                                        \
>>>>>>>       L1_DISALLOW_MASK : (L1_DISALLOW_MASK & ~PAGE_CACHE_ATTRS))
>>>>>>
>>>>>> ... cachability, which goes hand in hand with the ability to also
>>>>>> flush cache contents.
>>>>>
>>>>> Hm, I was on the edge here, in fact I've previously coded this using
>>>>> cache_flush_permitted(), just to the change back to
>>>>> has_arch_io_resources().  If you think cache_flush_permitted() is
>>>>> better I'm fine with that.
>>>>
>>>> I think that would be better here, yet as you say - it's not entirely
>>>> clear cut either way.
>>>
>>> I've reverted this chunk of the change and left the code as-is for the
>>> time being.
>>
>> Didn't we agree to use cache_flush_permitted() here instead?
> 
> I think it would be a bit weird, if we want this to be a
> non-functional change we would need to keep the has_arch_pdevs()
> condition because cache_flush_permitted() doesn't take that into
> account.  Or we need to adjust cache_flush_permitted() to also take
> has_arch_pdevs() into consideration.

Which is what you suggested elsewhere, or did I misunderstand that?

Jan