Re: XC_PAGE_SIZE or XEN_PAGE_SIZE?

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 12/02/2016 01:53 PM, Andrew Cooper wrote:
> On 02/12/16 11:43, Jan Beulich wrote:
>>>>> On 02.12.16 at 12:20, <andr2000@gmail.com> wrote:
>>> On 12/02/2016 01:03 PM, Jan Beulich wrote:
>>>>>>> On 02.12.16 at 08:49, <andr2000@gmail.com> 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
exactly for this reason I am defining

#define XENDISPL_PAGE_SIZE 4096
but I would probably change its name to XENDISPL_RING_SIZE



_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel