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Subject: Re: [Xen-devel] [PATCH v2 5/9] xen/gntdev: Allow mappings for DMA
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To:     Julien Grall <julien.grall@arm.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Oleksandr Andrushchenko <andr2000@gmail.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
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From:   Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
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Date:   Mon, 11 Jun 2018 20:49:58 +0300
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On 06/11/2018 08:46 PM, Julien Grall wrote:
> 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.
>
Sure, with a note that we waste most of a 64KiB page ;)
> Cheers,
>