Re: [Embedded-pv-devel] [PATCH v13] This is the ABI for the two halves of a para-virtualized sound driver to communicate with each to other.

[Julien Grall <julien.grall@xxxxxxx>]

Hi Oleksandr,

On 28/11/16 15:43, Oleksandr Andrushchenko wrote:
> On 11/28/2016 05:00 PM, Julien Grall wrote:
> > Hi Oleksandr,
> >
> > On 28/11/16 14:56, Oleksandr Andrushchenko wrote:
> > > On 11/28/2016 04:24 PM, Julien Grall wrote:
> > > > Hi Oleksandr,
> > > >
> > > > On 28/11/16 14:12, Oleksandr Andrushchenko wrote:
> > > > > On 11/28/2016 03:27 PM, Jan Beulich wrote:
> > > > > > > + *
> > > > > > > + * gref_dir_next_page - grant_ref_t, reference to the next page
> > > > > > > describing
> > > > > > > + *   page directory. Must be 0 if no more pages in the list.
> > > > If I am not mistaken 0 is a valid grant.
> > > Then I will remove this sentence, anyways BE knows how many grefs there
> > > are for the buffer size given
> BTW, xen-blkfrint.c:
> #define GRANT_INVALID_REF    0
> this is from where I got "Must be 0 if no more pages in the list."
> > > > > > > + * gref[i] - grant_ref_t, reference to a shared page of the buffer
> > > > > > > + *   allocated at XENSND_OP_OPEN
> > > > > > > + *
> > > > > > > + * Number of grant_ref_t entries in the whole page directory is not
> > > > > > > + * passed, but instead can be calculated as:
> > > > > > > + *   num_grefs_total = DIV_ROUND_UP(XENSND_OP_OPEN.buffer_sz,
> > > > > > > PAGE_SIZE);
> > > > > > The header should be self contained, and there's no DIV_ROUND_UP()
> > > > > > anywhere under public/io/ for a reader to refer to. Please express
> > > > > > this
> > > > > > using mathematical terms plus, if needed, standard C library ones.
> > > > > done, will put:
> > > > > num_grefs_total = (XENSND_OP_OPEN.buffer_sz + PAGE_SIZE - 1) /
> > > > > PAGE_SIZE
> > > > Can we avoid to use PAGE_SIZE in the header? Xen, the front-end and
> > > > the back-end may have different page size.
> > > then, I believe, the protocol should implement something like blkif
> > > does?
> > > Multi-page buffer which depends on front and back page sizes?
> > > (blkif: max-ring-page-order/ring-ref%u/ring-page-order)
> > > Is this what you mean?
> > It is not what I meant. I asked to define PAGE_SIZE. Is it the the
> > PAGE_SIZE of Xen? PAGE_SIZE of the backend? PAGE_SIZE of the frontend?
> >
> > Currently PV driver PAGE_SIZE is based on the size of a grant page.
> As per my understanding, I can put XEN_PAGE_SIZE here, because
> xen/page.h says:
> " * We assume that PAGE_SIZE is a multiple of XEN_PAGE_SIZE
>  * XXX: Add a BUILD_BUG_ON?
> "
> meaning that PAGE_SIZE on either side cannot be less than XEN_PAGE_SIZE
> Will XEN_PAGE_SIZE fit then?
Yes. FWIW, this is the page granularity used by every PV drivers.

Regards,

--
Julien Grall

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