Re: [PATCH] xen/mm: Remove loops in get_order_from_{pages,bytes}()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.10.2026 19:07, Andrew Cooper wrote:
> On 28/10/2026 2:15 pm, Jan Beulich wrote:
>> On 28.10.2026 13:41, Andrew Cooper wrote:
>>> --- a/xen/include/xen/mm.h
>>> +++ b/xen/include/xen/mm.h
>>> @@ -530,26 +530,16 @@ page_list_splice(struct page_list_head *list, struct 
>>> page_list_head *head)
>>>      list_for_each_entry_safe_reverse(pos, tmp, head, list)
>>>  #endif
>>>  
>>> -static inline unsigned int get_order_from_bytes(paddr_t size)
>>> +static always_inline unsigned int get_order_from_bytes(paddr_t size)
>>>  {
>>> -    unsigned int order;
>>> -
>>>      size = (size - 1) >> PAGE_SHIFT;
>>> -    for ( order = 0; size; order++ )
>>> -        size >>= 1;
>>>  
>>> -    return order;
>>> +    return fls64(size);
>>>  }
>> Along the lines of what you do below, perhaps simply
>>
>>     return fls_g(PFN_DOWN(size - 1));
>>
>> (and nothing else in the function body)? And then even
>>
>>     return flsl(PFN_DOWN(size - 1));
>>
>> (eliminating the immediate need for fls_g())?
> 
> Neither of these are quite correct.  fls64() can't be downgraded to
> flsl(), as it would truncate the input on 32bit builds of Xen.

Only hypothetically: get_order_from_pages() taking an unsigned long is
okay on 32-bit builds. In fact in principle get_order_from_bytes()
could be implemented by calling get_order_from_pages() with a suitably
adjusted argument.

Jan