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 16:13:13 +0300
From:   Oleksandr Andrushchenko <andr2000@...il.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "Oleksandr_Andrushchenko@...m.com" <Oleksandr_Andrushchenko@...m.com>
Cc:     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,
        julien.grall@....com
Subject: Re: [Xen-devel] [PATCH v2 5/9] xen/gntdev: Allow mappings for DMA
 buffers

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?
> Thanks for the explanation.
>
> -boris
Thank you,
Oleksandr