[Xen-devel] Re: Reversing the DMA handle

[Pete Zaitcev <zaitcev@xxxxxxxxxx>]

On Wed, 10 Jan 2007 11:04:01 +0000, Keir Fraser <keir@xxxxxxxxxxxxx> wrote:
> On 10/1/07 01:17, "Pete Zaitcev" <zaitcev@xxxxxxxxxx> wrote:

> > My question is: how to find PFN under Xen if DMA address is known,
> > on x86_64 and i386?
> 
> Use mfn_to_local_pfn() which is safe to call from any context. [...]
> Of course you may then want to do further translation if that happens to be
> in the swiotlb aperture.

Keir, thanks a lot, that worked perfectly.

The SWIOTLB is something I can't understand still. The code sure looks
like returning physical addresses to be used as DMA handles, just like
any usual routines do:

void *
swiotlb_alloc_coherent(struct device *hwdev, size_t size,
                       dma_addr_t *dma_handle, gfp_t flags)
{
....
        ret = (void *)__get_free_pages(flags, order);
        dev_addr = virt_to_phys(ret);
        *dma_handle = dev_addr;

And the code in pci-dma.c does:

void *
dma_alloc_coherent(struct device *dev, size_t size, dma_addr_t *dma_handle,
                   gfp_t gfp)
        memory = dma_alloc_pages(dev, gfp, get_order(size));
                        *dma_handle = virt_to_bus(memory);

The virt_to_bus and virt_to_phys are the same on x86_64.

So this is a kind of a mystery, but I'll think about it and maybe
ask at linux-kernel.

Thanks,
-- Pete

diff -urp -X dontdiff linux-2.6.17-1.2647.fc6/drivers/usb/mon/mon_dma.c 
linux-2.6.17-1.2647.fc6.z1/drivers/usb/mon/mon_dma.c
--- linux-2.6.17-1.2647.fc6/drivers/usb/mon/mon_dma.c   2006-09-24 
19:10:42.000000000 -0700
+++ linux-2.6.17-1.2647.fc6.z1/drivers/usb/mon/mon_dma.c        2007-01-10 
17:30:33.000000000 -0800
@@ -19,21 +19,25 @@
 #if defined(__i386__) || defined(__x86_64__) /* CONFIG_ARCH_I386 doesn't exit 
*/
 #define MON_HAS_UNMAP 1
 
-#define phys_to_page(phys)     pfn_to_page((phys) >> PAGE_SHIFT)
-
 char mon_dmapeek(unsigned char *dst, dma_addr_t dma_addr, int len)
 {
        struct page *pg;
        unsigned long flags;
        unsigned char *map;
        unsigned char *ptr;
+       unsigned long mfn;
 
        /*
         * On i386, a DMA handle is the "physical" address of a page.
         * In other words, the bus address is equal to physical address.
-        * There is no IOMMU.
+        * But sometimes IOMMUs are present (Calgary, AMD's x86_64, SWIOTLB).
+        * Also, Xen introduces "machine" address, which is used for DMA.
+        * We handle Xen, but not IOMMUs yet.
         */
-       pg = phys_to_page(dma_addr);
+       mfn = mfn_to_local_pfn(dma_addr >> PAGE_SHIFT);
+       if (!pfn_valid(mfn))
+               return 'X';
+       pg = pfn_to_page(mfn);
 
        /*
         * We are called from hardware IRQs in case of callbacks.

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