[Xen-devel] [PATCH 2/2] x86, xen: correct dma_get_required_mask() for Xen PV guests

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On systems where DMA addresses and physical addresses are not 1:1
(such as Xen PV guests), the generic dma_get_required_mask() will not
return the correct mask (since it uses max_pfn).

Some device drivers (such as mptsas, mpt2sas) use
dma_get_required_mask() to set device DMA masks 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 an arch-specific dma_get_required_mask() that defaults to the
generic dma_get_required_mask_from_pfn().

Under Xen, the required DMA mask can then be set to always 64-bits.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 arch/x86/include/asm/device.h  |    2 ++
 arch/x86/kernel/pci-dma.c      |    7 +++++++
 arch/x86/xen/pci-swiotlb-xen.c |    1 +
 drivers/xen/swiotlb-xen.c      |    7 +++++++
 include/xen/swiotlb-xen.h      |    4 ++++
 5 files changed, 21 insertions(+)

diff --git a/arch/x86/include/asm/device.h b/arch/x86/include/asm/device.h
index 03dd729..10bc628 100644
--- a/arch/x86/include/asm/device.h
+++ b/arch/x86/include/asm/device.h
@@ -13,4 +13,6 @@ struct dev_archdata {
 struct pdev_archdata {
 };
 
+#define ARCH_HAS_DMA_GET_REQUIRED_MASK
+
 #endif /* _ASM_X86_DEVICE_H */
diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
index f7d0672..ad2c3e2 100644
--- a/arch/x86/kernel/pci-dma.c
+++ b/arch/x86/kernel/pci-dma.c
@@ -135,6 +135,13 @@ void dma_generic_free_coherent(struct device *dev, size_t 
size, void *vaddr,
                free_pages((unsigned long)vaddr, get_order(size));
 }
 
+u64 dma_get_required_mask(struct device *dev)
+{
+       if (dma_ops->get_required_mask)
+               return dma_ops->get_required_mask(dev);
+       return dma_get_required_mask_from_max_pfn(dev);
+}
+
 /*
  * See <Documentation/x86/x86_64/boot-options.txt> for the iommu kernel
  * parameter documentation.
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..798f62b 100644
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c
@@ -683,3 +683,10 @@ 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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