Re: [Xen-devel] [PATCH v2] xen/arm: correctly handle DMA mapping of compound pages

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 8 Feb 2016, Ian Campbell wrote:
> Currently xen_dma_map_page concludes that DMA to anything other than
> the head page of a compound page must be foreign, since the PFN of the
> page is that of the head.
> 
> Fix the check to instead consider the whole of a compound page to be
> local if the PFN of the head passes the 1:1 check.
> 
> We can never see a compound page which is a mixture of foreign and
> local sub-pages.
> 
> The comment already correctly described the intention, but fixup the
> spelling and some grammar.
> 
> This fixes the various SSH protocol errors which we have been seeing
> on the cubietrucks in our automated test infrastructure.
> 
> This has been broken since commit 3567258d281b ("xen/arm: use
> hypercall to flush caches in map_page"), which was in v3.19-rc1.
> 
> NB arch/arm64/.../xen/page-coherent.h also includes this file.
> 
> Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx # v3.19+

Reviewed-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>

I'll add it to my queue.


> v2: Placate checkpatch
>     Handle 64K pages
>     Add some ()s
> ---
>  arch/arm/include/asm/xen/page-coherent.h | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm/include/asm/xen/page-coherent.h 
> b/arch/arm/include/asm/xen/page-coherent.h
> index 0375c8c..9408a99 100644
> --- a/arch/arm/include/asm/xen/page-coherent.h
> +++ b/arch/arm/include/asm/xen/page-coherent.h
> @@ -35,14 +35,21 @@ static inline void xen_dma_map_page(struct device *hwdev, 
> struct page *page,
>            dma_addr_t dev_addr, unsigned long offset, size_t size,
>            enum dma_data_direction dir, struct dma_attrs *attrs)
>  {
> -     bool local = XEN_PFN_DOWN(dev_addr) == page_to_xen_pfn(page);
> +     unsigned long page_pfn = page_to_xen_pfn(page);
> +     unsigned long dev_pfn = XEN_PFN_DOWN(dev_addr);
> +     unsigned long compound_pages =
> +             (1<<compound_order(page)) * XEN_PFN_PER_PAGE;
> +     bool local = (page_pfn <= dev_pfn) &&
> +             (dev_pfn - page_pfn < compound_pages);
> +
>       /*
> -      * Dom0 is mapped 1:1, while the Linux page can be spanned accross
> -      * multiple Xen page, it's not possible to have a mix of local and
> -      * foreign Xen page. So if the first xen_pfn == mfn the page is local
> -      * otherwise it's a foreign page grant-mapped in dom0. If the page is
> -      * local we can safely call the native dma_ops function, otherwise we
> -      * call the xen specific function.
> +      * Dom0 is mapped 1:1, while the Linux page can span across
> +      * multiple Xen pages, it's not possible for it to contain a
> +      * mix of local and foreign Xen pages. So if the first xen_pfn
> +      * == mfn the page is local otherwise it's a foreign page
> +      * grant-mapped in dom0. If the page is local we can safely
> +      * call the native dma_ops function, otherwise we call the xen
> +      * specific function.
>        */
>       if (local)
>               __generic_dma_ops(hwdev)->map_page(hwdev, page, offset, size, 
> dir, attrs);
> -- 
> 2.1.4
> 

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