Re: [PATCH 1/2] dma-mapping: Add dma_addr_is_phys_addr()

[Robin Murphy]

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 <bauer...@linux.ibm.com>
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 <b...@kernel.crashing.org>
cc: David Gibson <da...@gibson.dropbear.id.au>
cc: Michael Ellerman <m...@ellerman.id.au>
cc: Paul Mackerras <pau...@ozlabs.org>
cc: Michael Roth <mdr...@linux.vnet.ibm.com>
cc: Alexey Kardashevskiy <a...@linux.ibm.com>
cc: Paul Burton <paul.bur...@mips.com>
cc: Robin Murphy <robin.mur...@arm.com>
cc: Bartlomiej Zolnierkiewicz <b.zolnier...@samsung.com>
cc: Marek Szyprowski <m.szyprow...@samsung.com>
cc: Christoph Hellwig <h...@lst.de>
Suggested-by: Michael S. Tsirkin <m...@redhat.com>
Signed-off-by: Ram Pai <linux...@us.ibm.com>
Signed-off-by: Thiago Jung Bauermann <bauer...@linux.ibm.com>

The change itself looks ok, so

Reviewed-by: David Gibson <da...@gibson.dropbear.id.au>

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