linux-kernel - Re: [PATCH 07/11] swiotlb-xen: use the same foreign page check everywhere

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Date:   Thu, 29 Aug 2019 15:47:06 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Christoph Hellwig <hch@....de>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        xen-devel@...ts.xenproject.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/11] swiotlb-xen: use the same foreign page check
 everywhere

On Mon, 26 Aug 2019, Christoph Hellwig wrote:
> xen_dma_map_page uses a different and more complicated check for foreign
> pages than the other three cache maintainance helpers.  Switch it to the
> simpler pfn_valid method a well, and document the scheme with a single
> improved comment in xen_dma_map_page.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

Reviewed-by: Stefano Stabellini <sstabellini@...nel.org>


> ---
>  include/xen/arm/page-coherent.h | 31 +++++++++----------------------
>  1 file changed, 9 insertions(+), 22 deletions(-)
> 
> diff --git a/include/xen/arm/page-coherent.h b/include/xen/arm/page-coherent.h
> index 0e244f4fec1a..07c104dbc21f 100644
> --- a/include/xen/arm/page-coherent.h
> +++ b/include/xen/arm/page-coherent.h
> @@ -41,23 +41,17 @@ 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, unsigned long attrs)
>  {
> -	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);
> +	unsigned long pfn = PFN_DOWN(dev_addr);
>  
>  	/*
> -	 * 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.
> +	 * Dom0 is mapped 1:1, and while the Linux page can span across multiple
> +	 * Xen pages, it is not possible for it to contain a mix of local and
> +	 * foreign Xen pages.  Calling pfn_valid on a foreign mfn will always
> +	 * return false, so if pfn_valid returns true the pages is local and we
> +	 * can use the native dma-direct functions, otherwise we call the Xen
> +	 * specific version.
>  	 */
> -	if (local)
> +	if (pfn_valid(pfn))
>  		dma_direct_map_page(hwdev, page, offset, size, dir, attrs);
>  	else
>  		__xen_dma_map_page(hwdev, page, dev_addr, offset, size, dir, attrs);
> @@ -67,14 +61,7 @@ static inline void xen_dma_unmap_page(struct device *hwdev, dma_addr_t handle,
>  		size_t size, enum dma_data_direction dir, unsigned long attrs)
>  {
>  	unsigned long pfn = PFN_DOWN(handle);
> -	/*
> -	 * 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. Dom0 is mapped 1:1, so calling pfn_valid on a
> -	 * foreign mfn will always return false. If the page is local we can
> -	 * safely call the native dma_ops function, otherwise we call the xen
> -	 * specific function.
> -	 */
> +
>  	if (pfn_valid(pfn))
>  		dma_direct_unmap_page(hwdev, handle, size, dir, attrs);
>  	else
> -- 
> 2.20.1
>