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SubjectRe: [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@citrix.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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