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Subject: Re: [Xen-devel] [RFC 1/3] xen/balloon: Allow allocating DMA buffers
To:     Oleksandr Andrushchenko <andr2000@gmail.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org,
        jgross@suse.com, konrad.wilk@oracle.com
Cc:     daniel.vetter@intel.com, matthew.d.roper@intel.com,
        dongwon.kim@intel.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
References: <20180517082604.14828-1-andr2000@gmail.com>
 <20180517082604.14828-2-andr2000@gmail.com>
 <6a108876-19b7-49d0-3de2-9e10f984736c@oracle.com>
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From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <b177a327-6a73-bb77-c69b-bc0958a05532@oracle.com>
Date:   Mon, 21 May 2018 14:53:08 -0400
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On 05/21/2018 01:32 PM, Oleksandr Andrushchenko wrote:
> On 05/21/2018 07:35 PM, Boris Ostrovsky wrote:
>> 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@epam.com>
>>>> A commit message would be useful.
>>> Sure, v1 will have it
>>>>> Signed-off-by: Oleksandr Andrushchenko
>>>>> <oleksandr_andrushchenko@epam.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.
> Well, I hoped that it would be easier to maintain if I modify existing
> code
> to support both use-cases, but I am also ok to create new routines if
> this
> seems to be reasonable - please let me know
>>   So the question is --- would it make
>> sense to do all of this separately from the balloon driver?
> This can be done, but which driver will host this code then? If we
> move from
> the balloon driver, then this could go to either gntdev or grant-table.
> What's your preference?

A separate module?

Is there any use for this feature outside of your zero-copy DRM driver?

-boris