linux-kernel - Re: [PATCH 4/4] x86/xen: assume a 64-bit DMA mask is required

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Date:	Fri, 21 Nov 2014 11:43:53 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	<linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
	<xen-devel@...ts.xenproject.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 4/4] x86/xen: assume a 64-bit DMA mask is required

On Thu, 20 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.
> 
> We could base the DMA mask on the maximum MFN but:
> 
> a) The hypercall op to get the maximum MFN (XENMEM_maximum_ram_page)
> will truncate the result to an int in 32-bit guests.
> 
> b) Future uses of the IOMMU in Xen may map frames at bus addresses
> above the end of RAM.
> 
> So, just assume a 64-bit DMA mask is always required.
 
That's OK for me but at this point I am unsure whether swiotlb-xen is
the right place to put this function.  It feels more like something that
should be introduced in pci-swiotlb-xen.c or arch/x86/xen/mmu.c?

If I were to introduce it on ARM, I would add it to arch/arm/xen/mm.c.


> Signed-off-by: David Vrabel <david.vrabel@...rix.com>
>
>  arch/x86/xen/pci-swiotlb-xen.c |    1 +
>  drivers/xen/swiotlb-xen.c      |    9 +++++++++
>  include/xen/swiotlb-xen.h      |    4 ++++
>  3 files changed, 14 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..767d048 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 +685,10 @@ 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)
> +{
> +	return DMA_BIT_MASK(64);
> +}
> +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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