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SubjectRe: [PATCH v8 18/19] swiotlb-xen: no need to do execute two bus_to_phys translations in a row
On Thu, Oct 17, 2013 at 06:43:33PM +0100, Stefano Stabellini wrote:
> xen_unmap_single calls xen_bus_to_phys, then passes dev_addr to
> is_xen_swiotlb_buffer that calls xen_bus_to_phys again.

It does?

I see it call mfn_to_local_pfn which calls pfn_to_mfn and
get_phys_to_machine (which I presume is going to be a slow
operation on ARM).

> xen_bus_to_phys is potentially a slow operation that might involve
> walking a tree.
> We can avoid calling xen_bus_to_phys twice by removing the
> is_xen_swiotlb_buffer call and doing the check directly in
> xen_unmap_single using the physical address.
>
> Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

Have you tested this with a PFN that is in a foreign domain?

I am bit uncomfortable with this - I recall when developing this
that I had hit some odd cases of doing DMA of a page to a guest
and it not always working. It was the case of not detecting whether
the pfn was foreign or not. Sadly I can't recall the details.

I would think that this problem would also be present with ARM?
But perhaps all the new changes in the pfn_to_mfn that had lately
been done have fixed this?

> ---
> drivers/xen/swiotlb-xen.c | 4 ++--
> 1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index 4d50058..6d9ca61 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -426,8 +426,8 @@ static void xen_unmap_single(struct device *hwdev, dma_addr_t dev_addr,
>
> xen_dma_unmap_page(hwdev, paddr, size, dir, attrs);
>
> - /* NOTE: We use dev_addr here, not paddr! */
> - if (is_xen_swiotlb_buffer(dev_addr)) {
> + if (paddr >= virt_to_phys(xen_io_tlb_start) &&
> + paddr < virt_to_phys(xen_io_tlb_end)) {
> swiotlb_tbl_unmap_single(hwdev, paddr, size, dir);
> return;
> }
> --
> 1.7.2.5
>


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