Re: [Xen-devel] XC_PAGE_SIZE or XEN_PAGE_SIZE?

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/12/16 11:43, Jan Beulich wrote:
>>>> On 02.12.16 at 12:20, <andr2000@xxxxxxxxx> wrote:
>> On 12/02/2016 01:03 PM, Jan Beulich wrote:
>>>>>> On 02.12.16 at 08:49, <andr2000@xxxxxxxxx> wrote:
>>>> While working on display protocol I found that there is no(?) common
>>>>
>>>> ???_PAGE_SIZE define I can use for both Xen and Linux kernel:
>>>>
>>>> Xen defines XC_PAGE_SIZE which is also used in Linux user-space and
>>>>
>>>> kernel has XEN_PAGE_SIZE, but no XC_PAGE_SIZE.
>>>>
>>>> So, the question is which define should I use?
>>> In the abstract protocol there should be no need for this other
>>> than in comments (as is the case for all other protocols with the
>>> exception of vscsiif, which had a need to define its own
>>> VSCSIIF_PAGE_SIZE), where I think just PAGE_SIZE will be fine.
>>> In the end, frontend and backend are required to agree on a
>>> page size via some side channel anyway, which usually is
>>> achieved by base architecture assumptions (after all both run
>>> on the same physical machine and hence with the same set of
>>> architecture prerequisites). Arguably this is not an optimal model
>>> (namely on architectures supporting varying page sizes), but if
>>> we were to change it we should probably do so for all protocols.
>>>
>>> In the implementation you use the manifest constant available: If
>>> your code is in user space, use XC_PAGE_SIZE. In the kernel you'd
>>> obviously use XEN_PAGE_SIZE.
>> I'll give you an example:
>> #define XENDISPL_PAGE_SIZE 4096
>> #define XENDISPL_IN_RING_OFFS (sizeof(struct xendispl_event_page))
>> #define XENDISPL_IN_RING_SIZE (XENDISPL_PAGE_SIZE - XENDISPL_IN_RING_OFFS)
>>
>> By this code I define an event ring for async messages from front to back.
>> This is almost the same as already defined in kbdif and fbif which 
>> define it to 2048
>> In my case I wanted to rely on page size.
> But as said - from an abstract perspective there's no universal
> page size here, so I don't see why you want to make one up.

Particularly on ARM, you may have one domain using 4k pages and one
domain using 64k.

This issue has already been fudged once because of a blanket assumption
of the use of 4k pages.

All new work should have care taken to deal cleanly with the problem.

~Andrew

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