Re: [Xen-devel] [PATCH 4/4] x86/xen: use the maximum MFN to calculate the required DMA mask

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Wed, 19 Nov 2014, David Vrabel wrote:
> On a Xen PV guest the DMA addresses and physical addresses are not 1:1
> (such as Xen PV guests) and the generic dma_get_required_mask() does
> not return the correct mask (since it uses max_pfn).
> 
> Some device drivers (such as mptsas, mpt2sas) use
> dma_get_required_mask() to set the device's DMA mask to allow them to
> use only 32-bit DMA addresses in hardware structures.  This results in
> unnecessary use of the SWIOTLB if DMA addresses are more than 32-bits,
> impacting performance significantly.
> 
> Provide a get_required_mask op that uses the maximum MFN to calculate
> the DMA mask.
> 
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> ---
>  arch/x86/xen/pci-swiotlb-xen.c |    1 +
>  drivers/xen/swiotlb-xen.c      |   13 +++++++++++++
>  include/xen/swiotlb-xen.h      |    4 ++++
>  3 files changed, 18 insertions(+)
> 
> diff --git a/arch/x86/xen/pci-swiotlb-xen.c b/arch/x86/xen/pci-swiotlb-xen.c
> index 0e98e5d..a5d180a 100644
> --- a/arch/x86/xen/pci-swiotlb-xen.c
> +++ b/arch/x86/xen/pci-swiotlb-xen.c
> @@ -31,6 +31,7 @@ static struct dma_map_ops xen_swiotlb_dma_ops = {
>  	.map_page = xen_swiotlb_map_page,
>  	.unmap_page = xen_swiotlb_unmap_page,
>  	.dma_supported = xen_swiotlb_dma_supported,
> +	.get_required_mask = xen_swiotlb_get_required_mask,
>  };
>  
>  /*
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index ebd8f21..654587d 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -42,9 +42,11 @@
>  #include <xen/page.h>
>  #include <xen/xen-ops.h>
>  #include <xen/hvc-console.h>
> +#include <xen/interface/memory.h>
>  
>  #include <asm/dma-mapping.h>
>  #include <asm/xen/page-coherent.h>
> +#include <asm/xen/hypercall.h>
>  
>  #include <trace/events/swiotlb.h>
>  /*
> @@ -683,3 +685,14 @@ xen_swiotlb_set_dma_mask(struct device *dev, u64 dma_mask)
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(xen_swiotlb_set_dma_mask);
> +
> +u64
> +xen_swiotlb_get_required_mask(struct device *dev)
> +{
> +	unsigned long max_mfn;
> +
> +	max_mfn = HYPERVISOR_memory_op(XENMEM_maximum_ram_page, NULL);

As Jan pointed out, I think you need to change the prototype of
HYPERVISOR_memory_op to return long. Please do consistently across all
relevant archs.


> +	return DMA_BIT_MASK(fls_long(max_mfn - 1) + PAGE_SHIFT);
> +}
> +EXPORT_SYMBOL_GPL(xen_swiotlb_get_required_mask);
> diff --git a/include/xen/swiotlb-xen.h b/include/xen/swiotlb-xen.h
> index 8b2eb93..6408888 100644
> --- a/include/xen/swiotlb-xen.h
> +++ b/include/xen/swiotlb-xen.h
> @@ -58,4 +58,8 @@ xen_swiotlb_dma_supported(struct device *hwdev, u64 mask);
>  
>  extern int
>  xen_swiotlb_set_dma_mask(struct device *dev, u64 dma_mask);
> +
> +extern u64
> +xen_swiotlb_get_required_mask(struct device *dev);
> +
>  #endif /* __LINUX_SWIOTLB_XEN_H */
> -- 
> 1.7.10.4
> 
> 
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