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

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Message-ID: <55C8A648.20103@citrix.com>
Date:	Mon, 10 Aug 2015 14:25:28 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	<ian.campbell@...rix.com>, <linux-kernel@...r.kernel.org>,
	<xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH v3 19/20] xen/privcmd: Add support for Linux
 64KB page granularity

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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