linux-kernel - Re: [Xen-devel] [PATCH v2 5/9] xen/gntdev: Allow mappings for DMA buffers

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Date:   Mon, 11 Jun 2018 18:46:45 +0100
From:   Julien Grall <julien.grall@....com>
To:     Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Andrushchenko <andr2000@...il.com>
Cc:     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, daniel.vetter@...el.com,
        matthew.d.roper@...el.com, dongwon.kim@...el.com
Subject: Re: [Xen-devel] [PATCH v2 5/9] xen/gntdev: Allow mappings for DMA
 buffers

Hi,

On 06/11/2018 06:16 PM, Oleksandr Andrushchenko wrote:
> On 06/11/2018 07:51 PM, Stefano Stabellini wrote:
>> On Mon, 11 Jun 2018, Oleksandr Andrushchenko wrote:
>>> On 06/08/2018 10:21 PM, Boris Ostrovsky wrote:
>>>> On 06/08/2018 01:59 PM, Stefano Stabellini wrote:
>>>>>>>>>>>>       @@ -325,6 +401,14 @@ static int map_grant_pages(struct
>>>>>>>>>>>> grant_map
>>>>>>>>>>>> *map)
>>>>>>>>>>>>               map->unmap_ops[i].handle =
>>>>>>>>>>>> map->map_ops[i].handle;
>>>>>>>>>>>>               if (use_ptemod)
>>>>>>>>>>>>                   map->kunmap_ops[i].handle =
>>>>>>>>>>>> map->kmap_ops[i].handle;
>>>>>>>>>>>> +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
>>>>>>>>>>>> +        else if (map->dma_vaddr) {
>>>>>>>>>>>> +            unsigned long mfn;
>>>>>>>>>>>> +
>>>>>>>>>>>> +            mfn = __pfn_to_mfn(page_to_pfn(map->pages[i]));
>>>>>>>>>>> Not pfn_to_mfn()?
>>>>>>>>>> I'd love to, but pfn_to_mfn is only defined for x86, not ARM:
>>>>>>>>>> [1]
>>>>>>>>>> and [2]
>>>>>>>>>> Thus,
>>>>>>>>>>
>>>>>>>>>> drivers/xen/gntdev.c:408:10: error: implicit declaration of
>>>>>>>>>> function
>>>>>>>>>> ‘pfn_to_mfn’ [-Werror=implicit-function-declaration]
>>>>>>>>>>         mfn = pfn_to_mfn(page_to_pfn(map->pages[i]));
>>>>>>>>>>
>>>>>>>>>> So, I'll keep __pfn_to_mfn
>>>>>>>>> How will this work on non-PV x86?
>>>>>>>> So, you mean I need:
>>>>>>>> #ifdef CONFIG_X86
>>>>>>>> mfn = pfn_to_mfn(page_to_pfn(map->pages[i]));
>>>>>>>> #else
>>>>>>>> mfn = __pfn_to_mfn(page_to_pfn(map->pages[i]));
>>>>>>>> #endif
>>>>>>>>
>>>>>>> I'd rather fix it in ARM code. Stefano, why does ARM uses the
>>>>>>> underscored version?
>>>>>> Do you want me to add one more patch for ARM to wrap __pfn_to_mfn
>>>>>> with static inline for ARM? e.g.
>>>>>> static inline ...pfn_to_mfn(...)
>>>>>> {
>>>>>>       __pfn_to_mfn();
>>>>>> }
>>>>> A Xen on ARM guest doesn't actually know the mfns behind its own
>>>>> pseudo-physical pages. This is why we stopped using pfn_to_mfn and
>>>>> started using pfn_to_bfn instead, which will generally return "pfn",
>>>>> unless the page is a foreign grant. See include/xen/arm/page.h.
>>>>> pfn_to_bfn was also introduced on x86. For example, see the usage of
>>>>> pfn_to_bfn in drivers/xen/swiotlb-xen.c. Otherwise, if you don't care
>>>>> about other mapped grants, you can just use pfn_to_gfn, that always
>>>>> returns pfn.
>>>> I think then this code needs to use pfn_to_bfn().
>>> Ok
>>>>
>>>>> Also, for your information, we support different page granularities in
>>>>> Linux as a Xen guest, see the comment at include/xen/arm/page.h:
>>>>>
>>>>>     /*
>>>>>      * The pseudo-physical frame (pfn) used in all the helpers is 
>>>>> always
>>>>> based
>>>>>      * on Xen page granularity (i.e 4KB).
>>>>>      *
>>>>>      * A Linux page may be split across multiple non-contiguous Xen 
>>>>> page so
>>>>> we
>>>>>      * have to keep track with frame based on 4KB page granularity.
>>>>>      *
>>>>>      * PV drivers should never make a direct usage of those helpers
>>>>> (particularly
>>>>>      * pfn_to_gfn and gfn_to_pfn).
>>>>>      */
>>>>>
>>>>> A Linux page could be 64K, but a Xen page is always 4K. A granted page
>>>>> is also 4K. We have helpers to take into account the offsets to map
>>>>> multiple Xen grants in a single Linux page, see for example
>>>>> drivers/xen/grant-table.c:gnttab_foreach_grant. Most PV drivers have
>>>>> been converted to be able to work with 64K pages correctly, but if I
>>>>> remember correctly gntdev.c is the only remaining driver that doesn't
>>>>> support 64K pages yet, so you don't have to deal with it if you don't
>>>>> want to.
>>>> I believe somewhere in this series there is a test for PAGE_SIZE vs.
>>>> XEN_PAGE_SIZE. Right, Oleksandr?
>>> Not in gntdev. You might have seen this in xen-drmfront/xen-sndfront,
>>> but I didn't touch gntdev for that. Do you want me to add yet another 
>>> patch
>>> in the series to check for that?
>> gntdev.c is already not capable of handling PAGE_SIZE != XEN_PAGE_SIZE,
>> so you are not going to break anything that is not already broken :-) If
>> your new gntdev.c code relies on PAGE_SIZE == XEN_PAGE_SIZE, it might be
>> good to add an in-code comment about it, just to make it easier to fix
>> the whole of gntdev.c in the future.
>>
> Yes, I just mean I can add something like [1] as a separate patch to the 
> series,
> so we are on the safe side here

See my comment on Stefano's e-mail. I believe gntdev is able to handle 
PAGE_SIZE != XEN_PAGE_SIZE. So I would rather keep the behavior we have 
today for such case.

Cheers,

-- 
Julien Grall