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

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Date:   Mon, 21 May 2018 20:32:15 +0300
From:   Oleksandr Andrushchenko <andr2000@...il.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.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 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@...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.
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?
>
> -boris
Thank you,
Oleksandr