Re: [Linux] [ARM] Granting memory obtained from the DMA API

[Simon Leiner <simon@xxxxxxxxx>]

On 20.08.20 20:35, Stefano Stabellini wrote:
> Thank for the well-written analysis of the problem. The following 
should
> work to translate the virtual address properly in xenbus_grant_ring:
> 
>       if (is_vmalloc_addr(vaddr))
>               page = vmalloc_to_page(vaddr);
>       else
>               page = virt_to_page(vaddr);

Great idea, thanks! I modified it lightly (see below) and it did indeed 
work! I'm wondering though whether the check for vmalloc'd addresses 
should be included directly in the ARM implementation of virt_to_gfn. 
As far as I see, this should not break anything, but might impose a 
small performance overhead in cases where it is known for sure that we 
are dealing with directly mapped memory. What do you think?

diff --git a/drivers/xen/xenbus/xenbus_client.c 
b/drivers/xen/xenbus/xenbus_client.c
index e17ca8156171..d7a97f946f2f 100644
--- a/drivers/xen/xenbus/xenbus_client.c
+++ b/drivers/xen/xenbus/xenbus_client.c
@@ -344,6 +344,21 @@ static void xenbus_switch_fatal(struct xenbus_device *dev, 
int depth, int err,
        __xenbus_switch_state(dev, XenbusStateClosing, 1);
 }
 
+/**
+ * vaddr_to_gfn
+ * @vaddr: any virtual address
+ * 
+ * Returns the guest frame number (GFN) corresponding to vaddr.
+ */
+static inline unsigned long vaddr_to_gfn(void *vaddr)
+{
+   if (is_vmalloc_addr(vaddr)) {
+       return pfn_to_gfn(vmalloc_to_pfn(vaddr));
+   } else {
+       return virt_to_gfn(vaddr);
+   }
+}
+
 /**
  * xenbus_grant_ring
  * @dev: xenbus device
@@ -364,7 +379,7 @@ int xenbus_grant_ring(struct xenbus_device *dev, void 
*vaddr,
 
    for (i = 0; i < nr_pages; i++) {
        err = gnttab_grant_foreign_access(dev->otherend_id,
-                         virt_to_gfn(vaddr), 0);
+                         vaddr_to_gfn(vaddr), 0);
        if (err < 0) {
            xenbus_dev_fatal(dev, err,
                     "granting access to ring page");