Re: [Xen-devel] [PATCH 05 of 14] Don't trigger unnecessary shadow scans on p2m entry update

["Jan Beulich" <JBeulich@xxxxxxxx>]

 >>> On 24.11.11 at 10:48, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>>>> On 23.11.11 at 22:11, Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx> wrote:
>> xen/arch/x86/mm/shadow/common.c |  4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>> 
>> 
>> When updating a p2m entry, the hypervisor scans all shadow pte's to find
>> mappings of that gfn and tear them down. This is avoided if the page count
>> reveals that there are no additional mappings. The current test ignores the
>> PGC_allocated flag and its effect on the page count.
>> 
>> Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
>> Signed-off-by: Adin Scannell <adin@xxxxxxxxxxx>
>> 
>> diff -r 93066bdc1e1c -r e8f0709af2b7 xen/arch/x86/mm/shadow/common.c
>> --- a/xen/arch/x86/mm/shadow/common.c
>> +++ b/xen/arch/x86/mm/shadow/common.c
>> @@ -2501,7 +2501,8 @@ int sh_remove_all_mappings(struct vcpu *
>>          ;
>>  
>>      perfc_incr(shadow_mappings);
>> -    if ( (page->count_info & PGC_count_mask) == 0 )
>> +    expected_count = (page->count_info & PGC_allocated) ? 1 : 0;
>> +    if ( (page->count_info & PGC_count_mask) == expected_count )
> 
> Is that really valid outside the locked region?
> 
>>          return 0;
>>  
>>      /* Although this is an externally visible function, we do not know
>> @@ -2517,7 +2518,6 @@ int sh_remove_all_mappings(struct vcpu *
>>      hash_foreach(v, callback_mask, callbacks, gmfn);
>>  
>>      /* If that didn't catch the mapping, something is very wrong */
>> -    expected_count = (page->count_info & PGC_allocated) ? 1 : 0;
> 
> This certainly isn't right - iiuc the count would normally have changed
> during the hash_foreach() above this.

Hmm, I was wrong here, you aren't caching the page count. Still, is it
certain that the state of PGC_allocated doesn't change from where
you set expected_count now to where it was set before?

Jan


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