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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Also, for clarity the function should probably return bool_t.

Jan


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