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

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

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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