linux-kernel - Re: [Xen-devel] [RFC 1/3] xen/balloon: Allow allocating DMA buffers

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Date:   Mon, 21 May 2018 12:35:41 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Oleksandr Andrushchenko <andr2000@...il.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        jgross@...e.com, konrad.wilk@...cle.com
Cc:     daniel.vetter@...el.com, matthew.d.roper@...el.com,
        dongwon.kim@...el.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Subject: Re: [Xen-devel] [RFC 1/3] xen/balloon: Allow allocating DMA buffers

On 05/21/2018 01:40 AM, Oleksandr Andrushchenko wrote:
> On 05/19/2018 01:04 AM, Boris Ostrovsky wrote:
>> On 05/17/2018 04:26 AM, Oleksandr Andrushchenko wrote:
>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
>>
>> A commit message would be useful.
> Sure, v1 will have it
>>
>>> Signed-off-by: Oleksandr Andrushchenko
>>> <oleksandr_andrushchenko@...m.com>
>>>
>>>       for (i = 0; i < nr_pages; i++) {
>>> -        page = alloc_page(gfp);
>>> -        if (page == NULL) {
>>> -            nr_pages = i;
>>> -            state = BP_EAGAIN;
>>> -            break;
>>> +        if (ext_pages) {
>>> +            page = ext_pages[i];
>>> +        } else {
>>> +            page = alloc_page(gfp);
>>> +            if (page == NULL) {
>>> +                nr_pages = i;
>>> +                state = BP_EAGAIN;
>>> +                break;
>>> +            }
>>>           }
>>>           scrub_page(page);
>>>           list_add(&page->lru, &pages);
>>> @@ -529,7 +565,7 @@ static enum bp_state
>>> decrease_reservation(unsigned long nr_pages, gfp_t gfp)
>>>       i = 0;
>>>       list_for_each_entry_safe(page, tmp, &pages, lru) {
>>>           /* XENMEM_decrease_reservation requires a GFN */
>>> -        frame_list[i++] = xen_page_to_gfn(page);
>>> +        frames[i++] = xen_page_to_gfn(page);
>>>     #ifdef CONFIG_XEN_HAVE_PVMMU
>>>           /*
>>> @@ -552,18 +588,22 @@ static enum bp_state
>>> decrease_reservation(unsigned long nr_pages, gfp_t gfp)
>>>   #endif
>>>           list_del(&page->lru);
>>>   -        balloon_append(page);
>>> +        if (!ext_pages)
>>> +            balloon_append(page);
>>
>> So what you are proposing is not really ballooning. You are just
>> piggybacking on existing interfaces, aren't you?
> Sort of. Basically I need to {increase|decrease}_reservation, not
> actually
> allocating ballooned pages.
> Do you think I can simply EXPORT_SYMBOL for
> {increase|decrease}_reservation?
> Any other suggestion?


I am actually wondering how much of that code you end up reusing. You
pretty much create new code paths in both routines and common code ends
up being essentially the hypercall. So the question is --- would it make
sense to do all of this separately from the balloon driver?


-boris