Re: [PATCH 3/8] xen/grant-table: Allow allocating buffers suitable for DMA

[Oleksandr Andrushchenko]

On 05/30/2018 06:20 PM, Boris Ostrovsky wrote:
> On 05/30/2018 02:34 AM, Oleksandr Andrushchenko wrote:
> > On 05/29/2018 10:10 PM, Boris Ostrovsky wrote:
> > > On 05/25/2018 11:33 AM, Oleksandr Andrushchenko wrote:
> > > +/**
> > > + * gnttab_dma_free_pages - free DMAable pages
> > > + * @args: arguments to the function
> > > + */
> > > +int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
> > > +{
> > > +ÂÂÂ xen_pfn_t *frames;
> > > +ÂÂÂ size_t size;
> > > +ÂÂÂ int i, ret;
> > > +
> > > +ÂÂÂ gnttab_pages_clear_private(args->nr_pages, args->pages);
> > > +
> > > +ÂÂÂ frames = kcalloc(args->nr_pages, sizeof(*frames), GFP_KERNEL);
> > >
> > > Any way you can do it without allocating memory? One possibility is to
> > > keep allocated frames from gnttab_dma_alloc_pages(). (Not sure I like
> > > that either but it's the only thing I can think of).
> > Yes, I was also thinking about storing the allocated frames array from
> > gnttab_dma_alloc_pages(), but that seemed not to be clear enough as
> > the caller of the gnttab_dma_alloc_pages will need to store those frames
> > in some context, so we can pass them on free. But the caller doesn't
> > really
> > need the frames which might confuse, so I decided to make those
> > allocations
> > on the fly.
> > But I can still rework that to store the frames if you insist: please
> > let me know.
>
> I would prefer not to allocate anything in the release path. Yes, I
> realize that dragging frames array around is not necessary but IMO it's
> better than potentially failing an allocation during a teardown. A
> comment in the struct definition could explain the reason for having
> this field.
Then I would suggest we have it this way: current API requires that
struct page **pages are allocated from outside. So, let's allocate
the frames from outside as well. This way the caller is responsible for
both pages and frames arrays and API looks consistent.
> > > > +ÂÂÂ if (!frames)
> > > > +ÂÂÂÂÂÂÂ return -ENOMEM;
> > > > +
> > > > +ÂÂÂ for (i = 0; i < args->nr_pages; i++)
> > > > +ÂÂÂÂÂÂÂ frames[i] = page_to_xen_pfn(args->pages[i]);
> > > Not xen_page_to_gfn()?
> > Well, according to [1] it should be :
> >  ÂÂÂ /* XENMEM_populate_physmap requires a PFN based on Xen
> >  ÂÂÂÂ * granularity.
> >  ÂÂÂÂ */
> >  ÂÂÂ frame_list[i] = page_to_xen_pfn(page);
>
> Ah, yes. I was looking at decrease_reservation and automatically assumed
> the same parameter type.
Good, then this one is resolved
> -boris
Thank you,
Oleksandr