Re: [PATCH 1/2] dma-mapping: Add dma_addr_is_phys_addr()
From: Robin Murphy
Date: Mon Oct 14 2019 - 06:32:06 EST
On 14/10/2019 05:51, David Gibson wrote:
On Fri, Oct 11, 2019 at 06:25:18PM -0700, Ram Pai wrote:
From: Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxx>
In order to safely use the DMA API, virtio needs to know whether DMA
addresses are in fact physical addresses and for that purpose,
dma_addr_is_phys_addr() is introduced.
cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
cc: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
cc: Paul Mackerras <paulus@xxxxxxxxxx>
cc: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
cc: Alexey Kardashevskiy <aik@xxxxxxxxxxxxx>
cc: Paul Burton <paul.burton@xxxxxxxx>
cc: Robin Murphy <robin.murphy@xxxxxxx>
cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
cc: Christoph Hellwig <hch@xxxxxx>
Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Ram Pai <linuxram@xxxxxxxxxx>
Signed-off-by: Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxx>
The change itself looks ok, so
Reviewed-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
However, I would like to see the commit message (and maybe the inline
comments) expanded a bit on what the distinction here is about. Some
of the text from the next patch would be suitable, about DMA addresses
usually being in a different address space but not in the case of
bounce buffering.
Right, this needs a much tighter definition. "DMA address happens to be
a valid physical address" is true of various IOMMU setups too, but I
can't believe it's meaningful in such cases.
If what you actually want is "DMA is direct or SWIOTLB" - i.e. "DMA
address is physical address of DMA data (not necessarily the original
buffer)" - wouldn't dma_is_direct() suffice?
Robin.
---
arch/powerpc/include/asm/dma-mapping.h | 21 +++++++++++++++++++++
arch/powerpc/platforms/pseries/Kconfig | 1 +
include/linux/dma-mapping.h | 20 ++++++++++++++++++++
kernel/dma/Kconfig | 3 +++
4 files changed, 45 insertions(+)
diff --git a/arch/powerpc/include/asm/dma-mapping.h b/arch/powerpc/include/asm/dma-mapping.h
index 565d6f7..f92c0a4b 100644
--- a/arch/powerpc/include/asm/dma-mapping.h
+++ b/arch/powerpc/include/asm/dma-mapping.h
@@ -5,6 +5,8 @@
#ifndef _ASM_DMA_MAPPING_H
#define _ASM_DMA_MAPPING_H
+#include <asm/svm.h>
+
static inline const struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
{
/* We don't handle the NULL dev case for ISA for now. We could
@@ -15,4 +17,23 @@ static inline const struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
return NULL;
}
+#ifdef CONFIG_ARCH_HAS_DMA_ADDR_IS_PHYS_ADDR
+/**
+ * dma_addr_is_phys_addr - check whether a device DMA address is a physical
+ * address
+ * @dev: device to check
+ *
+ * Returns %true if any DMA address for this device happens to also be a valid
+ * physical address (not necessarily of the same page).
+ */
+static inline bool dma_addr_is_phys_addr(struct device *dev)
+{
+ /*
+ * Secure guests always use the SWIOTLB, therefore DMA addresses are
+ * actually the physical address of the bounce buffer.
+ */
+ return is_secure_guest();
+}
+#endif
+
#endif /* _ASM_DMA_MAPPING_H */
diff --git a/arch/powerpc/platforms/pseries/Kconfig b/arch/powerpc/platforms/pseries/Kconfig
index 9e35cdd..0108150 100644
--- a/arch/powerpc/platforms/pseries/Kconfig
+++ b/arch/powerpc/platforms/pseries/Kconfig
@@ -152,6 +152,7 @@ config PPC_SVM
select SWIOTLB
select ARCH_HAS_MEM_ENCRYPT
select ARCH_HAS_FORCE_DMA_UNENCRYPTED
+ select ARCH_HAS_DMA_ADDR_IS_PHYS_ADDR
help
There are certain POWER platforms which support secure guests using
the Protected Execution Facility, with the help of an Ultravisor
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index f7d1eea..6df5664 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -693,6 +693,26 @@ static inline bool dma_addressing_limited(struct device *dev)
dma_get_required_mask(dev);
}
+#ifndef CONFIG_ARCH_HAS_DMA_ADDR_IS_PHYS_ADDR
+/**
+ * dma_addr_is_phys_addr - check whether a device DMA address is a physical
+ * address
+ * @dev: device to check
+ *
+ * Returns %true if any DMA address for this device happens to also be a valid
+ * physical address (not necessarily of the same page).
+ */
+static inline bool dma_addr_is_phys_addr(struct device *dev)
+{
+ /*
+ * Except in very specific setups, DMA addresses exist in a different
+ * address space from CPU physical addresses and cannot be directly used
+ * to reference system memory.
+ */
+ return false;
+}
+#endif
+
#ifdef CONFIG_ARCH_HAS_SETUP_DMA_OPS
void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
const struct iommu_ops *iommu, bool coherent);
diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig
index 9decbba..6209b46 100644
--- a/kernel/dma/Kconfig
+++ b/kernel/dma/Kconfig
@@ -51,6 +51,9 @@ config ARCH_HAS_DMA_MMAP_PGPROT
config ARCH_HAS_FORCE_DMA_UNENCRYPTED
bool
+config ARCH_HAS_DMA_ADDR_IS_PHYS_ADDR
+ bool
+
config DMA_NONCOHERENT_CACHE_SYNC
bool