Re: [Xen-devel] XC_PAGE_SIZE or XEN_PAGE_SIZE?

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

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@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
exactly for this reason I am defining

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



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