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

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 06/08/2018 12:46 AM, Boris Ostrovsky wrote:
> (Stefano, question for you at the end)
>
> On 06/07/2018 02:39 AM, Oleksandr Andrushchenko wrote:
>> On 06/07/2018 12:19 AM, Boris Ostrovsky wrote:
>>> On 06/06/2018 04:14 AM, Oleksandr Andrushchenko wrote:
>>>> On 06/04/2018 11:12 PM, Boris Ostrovsky wrote:
>>>>> On 06/01/2018 07:41 AM, Oleksandr Andrushchenko wrote:
>>>>> @@ -121,8 +146,27 @@ static void gntdev_free_map(struct grant_map
>>>>> *map)
>>>>>         if (map == NULL)
>>>>>             return;
>>>>>     +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
>> *Option 1: kfree(map->frames);*
>>>>> +    if (map->dma_vaddr) {
>>>>> +        struct gnttab_dma_alloc_args args;
>>>>> +
>>>>> +        args.dev = map->dma_dev;
>>>>> +        args.coherent = map->dma_flags & GNTDEV_DMA_FLAG_COHERENT;
>>>>> +        args.nr_pages = map->count;
>>>>> +        args.pages = map->pages;
>>>>> +        args.frames = map->frames;
>>>>> +        args.vaddr = map->dma_vaddr;
>>>>> +        args.dev_bus_addr = map->dma_bus_addr;
>>>>> +
>>>>> +        gnttab_dma_free_pages(&args);
>> *Option 2: kfree(map->frames);*
>>>>> +    } else
>>>>> +#endif
>>>>>         if (map->pages)
>>>>>             gnttab_free_pages(map->count, map->pages);
>>>>> +
>>>>> +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
>>>>> +    kfree(map->frames);
>>>>> +#endif
>>>>>
>>>>> Can this be done under if (map->dma_vaddr) ?
>>>>>     In other words, is it
>>>>> possible for dma_vaddr to be NULL and still have unallocated frames
>>>>> pointer?
>>>> It is possible to have vaddr == NULL and frames != NULL as we
>>>> allocate frames outside of gnttab_dma_alloc_pages which
>>>> may fail. Calling kfree on NULL pointer is safe,
>>> I am not questioning safety of the code, I would like avoid another
>>> ifdef.
>> Ah, I now understand, so you are asking if we can have
>> that kfree(map->frames); in the place *Option 2* I marked above.
>> Unfortunately no: map->frames is allocated before we try to
>> allocate DMA memory, e.g. before dma_vaddr is set:
>> [...]
>>          add->frames = kcalloc(count, sizeof(add->frames[0]),
>>                        GFP_KERNEL);
>>          if (!add->frames)
>>              goto err;
>>
>> [...]
>>          if (gnttab_dma_alloc_pages(&args))
>>              goto err;
>>
>>          add->dma_vaddr = args.vaddr;
>> [...]
>> err:
>>      gntdev_free_map(add);
>>
>> So, it is possible to enter gntdev_free_map with
>> frames != NULL and dma_vaddr == NULL. Option 1 above cannot be used
>> as map->frames is needed for gnttab_dma_free_pages(&args);
>> and Option 2 cannot be used as frames != NULL and dma_vaddr == NULL.
>> Thus, I think that unfortunately we need that #ifdef.
>> Option 3 below can also be considered, but that seems to be not good
>> as we free resources in different places which looks inconsistent.
>
> I was only thinking of option 2. But if it is possible to have frames !=
> NULL and dma_vaddr == NULL then perhaps we indeed will have to live with
> the extra ifdef.
ok
>
>> Sorry if I'm still missing your point.
>>>> so
>>>> I see no reason to change this code.
>>>>>>         kfree(map->pages);
>>>>>>         kfree(map->grants);
>>>>>>         kfree(map->map_ops);
>>>>>> @@ -132,7 +176,8 @@ static void gntdev_free_map(struct grant_map
>>>>>> *map)
>>>>>>         kfree(map);
>>>>>>     }
>>>>>>     -static struct grant_map *gntdev_alloc_map(struct gntdev_priv
>>>>>> *priv, int count)
>>>>>> +static struct grant_map *gntdev_alloc_map(struct gntdev_priv *priv,
>>>>>> int count,
>>>>>> +                      int dma_flags)
>>>>>>     {
>>>>>>         struct grant_map *add;
>>>>>>         int i;
>>>>>> @@ -155,6 +200,37 @@ static struct grant_map
>>>>>> *gntdev_alloc_map(struct gntdev_priv *priv, int count)
>>>>>>             NULL == add->pages)
>>>>>>             goto err;
>>>>>>     +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
>>>>>> +    add->dma_flags = dma_flags;
>>>>>> +
>>>>>> +    /*
>>>>>> +     * Check if this mapping is requested to be backed
>>>>>> +     * by a DMA buffer.
>>>>>> +     */
>>>>>> +    if (dma_flags & (GNTDEV_DMA_FLAG_WC |
>>>>>> GNTDEV_DMA_FLAG_COHERENT)) {
>>>>>> +        struct gnttab_dma_alloc_args args;
>>>>>> +
>>>>>> +        add->frames = kcalloc(count, sizeof(add->frames[0]),
>>>>>> +                      GFP_KERNEL);
>>>>>> +        if (!add->frames)
>>>>>> +            goto err;
>>>>>> +
>>>>>> +        /* Remember the device, so we can free DMA memory. */
>>>>>> +        add->dma_dev = priv->dma_dev;
>>>>>> +
>>>>>> +        args.dev = priv->dma_dev;
>>>>>> +        args.coherent = dma_flags & GNTDEV_DMA_FLAG_COHERENT;
>>>>>> +        args.nr_pages = count;
>>>>>> +        args.pages = add->pages;
>>>>>> +        args.frames = add->frames;
>>>>>> +
>>>>>> +        if (gnttab_dma_alloc_pages(&args))
>> *Option 3: kfree(map->frames);*
>>>>>> +            goto err;
>>>>>> +
>>>>>> +        add->dma_vaddr = args.vaddr;
>>>>>> +        add->dma_bus_addr = args.dev_bus_addr;
>>>>>> +    } else
>>>>>> +#endif
>>>>>>         if (gnttab_alloc_pages(count, add->pages))
>>>>>>             goto err;
>>>>>>     @@ -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();
}
>
> -boris
>
Thank you,
Oleksandr