Re: XC_PAGE_SIZE or XEN_PAGE_SIZE?

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 12/02/2016 01:43 PM, 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.
no reason, but my will to have it 4K
> Possibly different OSes running in different VMs may not even
> have a way to agree on a common value, so you're better of
> using some arbitrarily chose value (which still may match the
> page size in most commonly used architectures) without trying
> to derive is from whatever someone may consider is the page
> size.
#define XENDISPL_PAGE_SIZE 4096
> If granting access to that space is involved, then referring
> to grant page size in a comment may be reasonable (albeit you
> will find that nothing there requires it to be 4k either - any
> other power of two below 64k should be fine in theory).
>
> Jan
>
sure

Thank you,
Oleksandr

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