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

[Oleksandr Andrushchenko]

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)
> > >  ÂÂÂÂÂ if (map == NULL)
> > >  ÂÂÂÂÂÂÂÂÂ return;
> > >  Â +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
*Option 1: kfree(map->frames);*
> > > +ÂÂÂ if (map->dma_vaddr) {
> > > +ÂÂÂÂÂÂÂ struct gnttab_dma_alloc_args args;
> > > +
> > > +ÂÂÂÂÂÂÂ args.dev = map->dma_dev;
> > > +ÂÂÂÂÂÂÂ args.coherent = map->dma_flags & GNTDEV_DMA_FLAG_COHERENT;
> > > +ÂÂÂÂÂÂÂ args.nr_pages = map->count;
> > > +ÂÂÂÂÂÂÂ args.pages = map->pages;
> > > +ÂÂÂÂÂÂÂ args.frames = map->frames;
> > > +ÂÂÂÂÂÂÂ args.vaddr = map->dma_vaddr;
> > > +ÂÂÂÂÂÂÂ args.dev_bus_addr = map->dma_bus_addr;
> > > +
> > > +ÂÂÂÂÂÂÂ gnttab_dma_free_pages(&args);
*Option 2: kfree(map->frames);*
> > > +ÂÂÂ } else
> > > +#endif
> > >  ÂÂÂÂÂ if (map->pages)
> > >  ÂÂÂÂÂÂÂÂÂ gnttab_free_pages(map->count, map->pages);
> > > +
> > > +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
> > > +ÂÂÂ kfree(map->frames);
> > > +#endif
> > >
> > > Can this be done under if (map->dma_vaddr) ?
> > >  Â 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:
[...]
ÂÂÂ ÂÂÂ add->frames = kcalloc(count, sizeof(add->frames[0]),
ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂÂÂ GFP_KERNEL);
ÂÂÂ ÂÂÂ if (!add->frames)
ÂÂÂ ÂÂÂ ÂÂÂ goto err;

[...]
ÂÂÂ ÂÂÂ if (gnttab_dma_alloc_pages(&args))
ÂÂÂ ÂÂÂ ÂÂÂ goto err;

ÂÂÂ ÂÂÂ add->dma_vaddr = args.vaddr;
[...]
err:
ÂÂÂ 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.
> > > >  ÂÂÂÂÂ kfree(map->pages);
> > > >  ÂÂÂÂÂ kfree(map->grants);
> > > >  ÂÂÂÂÂ kfree(map->map_ops);
> > > > @@ -132,7 +176,8 @@ static void gntdev_free_map(struct grant_map *map)
> > > >  ÂÂÂÂÂ kfree(map);
> > > >  Â }
> > > >  Â -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,
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ int dma_flags)
> > > >  Â {
> > > >  ÂÂÂÂÂ struct grant_map *add;
> > > >  ÂÂÂÂÂ int i;
> > > > @@ -155,6 +200,37 @@ static struct grant_map
> > > > *gntdev_alloc_map(struct gntdev_priv *priv, int count)
> > > >  ÂÂÂÂÂÂÂÂÂ NULL == add->pages)
> > > >  ÂÂÂÂÂÂÂÂÂ goto err;
> > > >  Â +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
> > > > +ÂÂÂ add->dma_flags = dma_flags;
> > > > +
> > > > +ÂÂÂ /*
> > > > +ÂÂÂÂ * Check if this mapping is requested to be backed
> > > > +ÂÂÂÂ * by a DMA buffer.
> > > > +ÂÂÂÂ */
> > > > +ÂÂÂ if (dma_flags & (GNTDEV_DMA_FLAG_WC | GNTDEV_DMA_FLAG_COHERENT)) {
> > > > +ÂÂÂÂÂÂÂ struct gnttab_dma_alloc_args args;
> > > > +
> > > > +ÂÂÂÂÂÂÂ add->frames = kcalloc(count, sizeof(add->frames[0]),
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ GFP_KERNEL);
> > > > +ÂÂÂÂÂÂÂ if (!add->frames)
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ goto err;
> > > > +
> > > > +ÂÂÂÂÂÂÂ /* Remember the device, so we can free DMA memory. */
> > > > +ÂÂÂÂÂÂÂ add->dma_dev = priv->dma_dev;
> > > > +
> > > > +ÂÂÂÂÂÂÂ args.dev = priv->dma_dev;
> > > > +ÂÂÂÂÂÂÂ args.coherent = dma_flags & GNTDEV_DMA_FLAG_COHERENT;
> > > > +ÂÂÂÂÂÂÂ args.nr_pages = count;
> > > > +ÂÂÂÂÂÂÂ args.pages = add->pages;
> > > > +ÂÂÂÂÂÂÂ args.frames = add->frames;
> > > > +
> > > > +ÂÂÂÂÂÂÂ if (gnttab_dma_alloc_pages(&args))
*Option 3: kfree(map->frames);*
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ goto err;
> > > > +
> > > > +ÂÂÂÂÂÂÂ add->dma_vaddr = args.vaddr;
> > > > +ÂÂÂÂÂÂÂ add->dma_bus_addr = args.dev_bus_addr;
> > > > +ÂÂÂ } else
> > > > +#endif
> > > >  ÂÂÂÂÂ if (gnttab_alloc_pages(count, add->pages))
> > > >  ÂÂÂÂÂÂÂÂÂ goto err;
> > > >  Â @@ -325,6 +401,14 @@ static int map_grant_pages(struct grant_map
> > > > *map)
> > > >  ÂÂÂÂÂÂÂÂÂ map->unmap_ops[i].handle = map->map_ops[i].handle;
> > > >  ÂÂÂÂÂÂÂÂÂ if (use_ptemod)
> > > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂ map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
> > > > +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
> > > > +ÂÂÂÂÂÂÂ else if (map->dma_vaddr) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ unsigned long mfn;
> > > > +
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ 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
> > âpfn_to_mfnâ [-Werror=implicit-function-declaration]
> >  ÂÂÂ 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