Re: [Xen-devel] [PATCH v3 19/20] xen/privcmd: Add support for Linux 64KB page granularity

[Julien Grall]

Hi Stefano,

On 10/08/15 13:57, Stefano Stabellini wrote:
> On Mon, 10 Aug 2015, David Vrabel wrote:
>> On 10/08/15 13:03, Stefano Stabellini wrote:
>>> On Fri, 7 Aug 2015, Julien Grall wrote:
>>>> -	rc = HYPERVISOR_memory_op(XENMEM_add_to_physmap_range, &xatp);
>>>> -	return rc < 0 ? rc : err;
>>>> +	for (i = 0; i < nr_gfn; i++) {
>>>> +		if ((i % XEN_PFN_PER_PAGE) == 0) {
>>>> +			page = pages[i / XEN_PFN_PER_PAGE];
>>>
>>> If this function is going to be called very frequently you might want to
>>> consider using a shift instead.
>>>
>>>     page = pages[i >> 4];
>>>
>>> With an appropriate macro of course.
>>
>> This change isn't necessary.  Compilers already turn divides into
>> suitable shifts.
> 
> The ARM compiler I used last time I tested this did not, but that was 1
> or 2 years ago. In any case to be clear this change is not required.

I gave a try on the compiler used by Debian Jessy (gcc 4.9.2). It turns
divides into suitable shifts.

Anyway, if it may happen that older ARM compiler doesn't do this change,
I sure we would have to modify many other places in order to make the
code efficient.

Regards,

-- 
Julien Grall
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