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

Oleksandr Andrushchenko
2018-06-07T06:39:07Z

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.

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
> -boris
>
>
Thank you,
Oleksandr
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