Re: [PATCH] xen/swiotlb: Exchange to contiguous memory for map_sg hook

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Thu, Dec 06, 2012 at 09:08:42PM +0800, Dongxiao Xu wrote:
> While mapping sg buffers, checking to cross page DMA buffer is
> also needed. If the guest DMA buffer crosses page boundary, Xen
> should exchange contiguous memory for it.

So this is when we cross those 2MB contingous swatch of buffers.
Wouldn't we get the same problem with the 'map_page' call? If the
driver tried to map say a 4MB DMA region?

What if this check was done in the routines that provide the
software static buffers and there try to provide a nice
DMA contingous swatch of pages?

> 
> Besides, it is needed to backup the original page contents
> and copy it back after memory exchange is done.
> 
> This fixes issues if device DMA into software static buffers,
> and in case the static buffer cross page boundary which pages are
> not contiguous in real hardware.
> 
> Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
> Signed-off-by: Xiantao Zhang <xiantao.zhang@intel.com>
> ---
>  drivers/xen/swiotlb-xen.c |   47 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 files changed, 46 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index 58db6df..e8f0cfb 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -461,6 +461,22 @@ xen_swiotlb_sync_single_for_device(struct device *hwdev, dma_addr_t dev_addr,
>  }
>  EXPORT_SYMBOL_GPL(xen_swiotlb_sync_single_for_device);
>  
> +static bool
> +check_continguous_region(unsigned long vstart, unsigned long order)
> +{
> +	unsigned long prev_ma = xen_virt_to_bus((void *)vstart);
> +	unsigned long next_ma;
> +	int i;
> +
> +	for (i = 1; i < (1 << order); i++) {
> +		next_ma = xen_virt_to_bus((void *)(vstart + i * PAGE_SIZE));
> +		if (next_ma != prev_ma + PAGE_SIZE)
> +			return false;
> +		prev_ma = next_ma;
> +	}
> +	return true;
> +}
> +
>  /*
>   * Map a set of buffers described by scatterlist in streaming mode for DMA.
>   * This is the scatter-gather version of the above xen_swiotlb_map_page
> @@ -489,7 +505,36 @@ xen_swiotlb_map_sg_attrs(struct device *hwdev, struct scatterlist *sgl,
>  
>  	for_each_sg(sgl, sg, nelems, i) {
>  		phys_addr_t paddr = sg_phys(sg);
> -		dma_addr_t dev_addr = xen_phys_to_bus(paddr);
> +		unsigned long vstart, order;
> +		dma_addr_t dev_addr;
> +
> +		/*
> +		 * While mapping sg buffers, checking to cross page DMA buffer
> +		 * is also needed. If the guest DMA buffer crosses page
> +		 * boundary, Xen should exchange contiguous memory for it.
> +		 * Besides, it is needed to backup the original page contents
> +		 * and copy it back after memory exchange is done.
> +		 */
> +		if (range_straddles_page_boundary(paddr, sg->length)) {
> +			vstart = (unsigned long)__va(paddr & PAGE_MASK);
> +			order = get_order(sg->length + (paddr & ~PAGE_MASK));
> +			if (!check_continguous_region(vstart, order)) {
> +				unsigned long buf;
> +				buf = __get_free_pages(GFP_KERNEL, order);
> +				memcpy((void *)buf, (void *)vstart,
> +					PAGE_SIZE * (1 << order));
> +				if (xen_create_contiguous_region(vstart, order,
> +						fls64(paddr))) {
> +					free_pages(buf, order);
> +					return 0;
> +				}
> +				memcpy((void *)vstart, (void *)buf,
> +					PAGE_SIZE * (1 << order));
> +				free_pages(buf, order);
> +			}
> +		}
> +
> +		dev_addr = xen_phys_to_bus(paddr);
>  
>  		if (swiotlb_force ||
>  		    !dma_capable(hwdev, dev_addr, sg->length) ||
> -- 
> 1.7.1
>