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



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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