Re: [Xen-devel] [PATCH] Don't trigger unnecessary shadow scans on p2m entry update

[Keir Fraser <keir@xxxxxxx>]

On 25/11/2011 08:45, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

>>>> On 24.11.11 at 19:31, Keir Fraser <keir.xen@xxxxxxxxx> wrote:
>> On 24/11/2011 17:58, "Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx> wrote:
>>> @@ -815,6 +815,12 @@ static inline unsigned long vtlb_lookup(
>>>  }
>>>  #endif /* (SHADOW_OPTIMIZATIONS & SHOPT_VIRTUAL_TLB) */
>>>  
>>> +static inline int __check_page_no_refs(struct page_info *page)
>>> +{
>>> +    unsigned long __count = page->count_info;
>>> +    return ( (__count & PGC_count_mask) ==
>>> +             ((__count & PGC_allocated) ? 1 : 0) );
>> 
>> It's a fussy point, but it might be better to use
>> atomic_read_ulong(&page->count_info) here, to ensure that the compiler reads
>> the field once only. It's cheap (compiles to a single mov instruction), but
>> atomic_read_ulong doesn't exist so you'd have to add its (obvious)
>> definition to asm-x86/atomic.h
> 
> I think Tim suggested anyway to use
> 
> (__count & (PGC_count_mask|PGC_allocated)) matched against
> (1|PGC_allocated) here, which would eliminate the multiple read
> potential if used in a switch statement.

Yes, that would be better.

 -- Keir

> Also, for clarity the function should probably return bool_t.
> 
> Jan
> 



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