Re: [Xen-devel] [PATCH v7 2/9] mm: Place unscrubbed pages at the end of pagelist

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.08.17 at 16:41, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 08/15/2017 04:18 AM, Jan Beulich wrote:
>>>>> On 14.08.17 at 16:29, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 08/14/2017 06:37 AM, Jan Beulich wrote:
>>>>>>> On 08.08.17 at 23:45, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>> --- a/xen/include/asm-x86/mm.h
>>>>> +++ b/xen/include/asm-x86/mm.h
>>>>> @@ -88,7 +88,15 @@ struct page_info
>>>>>          /* Page is on a free list: ((count_info & PGC_count_mask) == 0). 
>>>>> */
>>>>>          struct {
>>>>>              /* Do TLBs need flushing for safety before next page use? */
>>>>> -            bool_t need_tlbflush;
>>>>> +            bool need_tlbflush:1;
>>>>> +
>>>>> +            /*
>>>>> +             * Index of the first *possibly* unscrubbed page in the 
>>>>> buddy.
>>>>> +             * One more bit than maximum possible order to accommodate
>>>>> +             * INVALID_DIRTY_IDX.
>>>>> +             */
>>>>> +#define INVALID_DIRTY_IDX ((1UL << (MAX_ORDER + 1)) - 1)
>>>>> +            unsigned long first_dirty:MAX_ORDER + 1;
>>>>>          } free;
>>>> I think generated code will be better with the two fields swapped:
>>>> That way reading first_dirty won't involve a shift, and accessing a
>>>> single bit doesn't require shifts at all on many architectures.
>>> Ok, I will then keep need_tlbflush as the last field so the final struct
>>> (as defined in patch 7) will look like
>>>
>>> struct {
>>>         unsigned long first_dirty:MAX_ORDER + 1;
>>>         unsigned long scrub_state:2;
>>>         bool need_tlbflush:1;
>>> };
>> Hmm, actually - why do you need bitfields on the x86 side at all?
>> They're needed for 32-bit architectures only, 64-bit ones ought
>> to be fine with
>>
>> struct {
>>         unsigned int first_dirty;
>>         bool need_tlbflush;
>>         uint8_t scrub_state;
>> };
> 
> IIRC it was exactly because of ARM32 and at some point you suggested to
> switch both x86 and ARM to bitfields.

I don't recall for sure whether I had asked for the change to be done
uniformly; it was certainly ARM32 that triggered me notice the
structure size change in your original version.

>> (plus a suitable BUILD_BUG_ON() to make sure first_dirty has
>> at least MAX_ORDER + 1 bits). The ARM maintainers will know
>> whether they would want to also differentiate ARM32 and
>> ARM64 here.
> 
> Isn't using bitfields the only possibility for 32-bit? We can't fit
> first_dirty into 2 bytes.

Yes, hence the question whether to stay with bitfields uniformly
or make ARM64 follow x86, but ARM32 keep using bitfields.

Jan


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