[PATCH 3/8] xen/grant-table: Allow allocating buffers suitable for DMA
andr2000 at gmail.com
Wed May 30 17:49:11 UTC 2018
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)
>>> + xen_pfn_t *frames;
>>> + size_t size;
>>> + int i, ret;
>>> + gnttab_pages_clear_private(args->nr_pages, args->pages);
>>> + 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
>> need the frames which might confuse, so I decided to make those
>> 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.
>>>> + if (!frames)
>>>> + return -ENOMEM;
>>>> + for (i = 0; i < args->nr_pages; i++)
>>>> + frames[i] = page_to_xen_pfn(args->pages[i]);
>>> Not xen_page_to_gfn()?
>> Well, according to  it should be :
>> /* XENMEM_populate_physmap requires a PFN based on Xen
>> * granularity.
>> 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
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