Re: [Xen-devel] [PATCH v5 3/8] arch, arm: let map_mmio_regions() take pfn as parameters

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Mon, 2014-04-07 at 01:31 +0200, Arianna Avanzini wrote:
> -        res = map_mmio_regions(d, addr & PAGE_MASK,
> -                               PAGE_ALIGN(addr + size) - 1,
> -                               addr & PAGE_MASK);
> +        res = map_mmio_regions(d,
> +                               paddr_to_pfn(addr & PAGE_MASK),
> +                               paddr_to_pfn_aligned(addr + size - 1),


With
+#define paddr_to_pfn_aligned(paddr)    paddr_to_pfn(PAGE_ALIGN(paddr))

There is a subtle difference here, which is that the "- 1" is now inside
the align. Does this always have the same result? I'm not sure.

If addr+size == 0x1000 (page aligned) then:

        PAGE_ALIGN(0x10000)-1 = 0x10000-1 = 0xffff

But

        paddr_to_pfn_aligned(0x10000 - 1) =
        paddr_to_pfn(PAGE_ALIGN(0xffff)) = paddr_to_pfn(0x10000) = 0x10

The new map_mmio_regions uses pfn_to_paddr which is:
#define pfn_to_paddr(pfn) ((paddr_t)(pfn) << PAGE_SHIFT)

So with the old code the end address would be 0xffff, while with the new
code it is 0x10<<12 = 0x10000.

I suspect the implementation of apply_to_p2m_changes is such that it
doesn't actually change anything. Can you confirm that this was your
intention?

Is the end argument tio map_mmio_regions now intended to be inclusive or
exclusive? This sort of issue can be avoided by using a count instead of
an end in the interface.

Ian.


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