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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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