[PATCH 0/2] extend support for CMA CMA on x86

[PATCH 0/2] extend support for CMA CMA on x86

[Akinobu Mita]

This patch set extends support for the DMA Contiguous Memory Allocator
on x86.  Currently it is only supported on pci-nommu.  So this aims at
enabling it on swiotlb and intel-iommu, too.

Regardless of which dma mapping implementation is actually used in the
system, I would like to allocate big contiguous memory with
dma_alloc_coherent() and tell the base address to the device that
requires it.  This is why I need this change.

Akinobu Mita (2):
  x86: enable DMA CMA with swiotlb
  intel-iommu: integrate DMA CMA

 arch/x86/Kconfig               |  2 +-
 arch/x86/include/asm/swiotlb.h |  7 +++++++
 arch/x86/kernel/amd_gart_64.c  |  2 +-
 arch/x86/kernel/pci-swiotlb.c  |  9 ++++++---
 arch/x86/pci/sta2x11-fixup.c   |  6 ++----
 drivers/iommu/intel-iommu.c    | 32 ++++++++++++++++++++++++--------
 include/linux/swiotlb.h        |  2 ++
 lib/swiotlb.c                  |  2 +-
 8 files changed, 44 insertions(+), 18 deletions(-)

Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: iommu@lists.linux-foundation.org
-- 
1.8.3.2

[PATCH 1/2] x86: enable DMA CMA with swiotlb

[Akinobu Mita]

The DMA Contiguous Memory Allocator support on x86 is disabled when
swiotlb config option is enabled.  So DMA CMA is always disabled on
x86_64 because swiotlb is always enabled.  This attempts to support
for DMA CMA with enabling swiotlb config option.

The contiguous memory allocator on x86 is integrated in the function
dma_generic_alloc_coherent() which is .alloc callback in nommu_dma_ops
for dma_alloc_coherent().

x86_swiotlb_alloc_coherent() which is .alloc callback in swiotlb_dma_ops
tries to allocate with dma_generic_alloc_coherent() firstly and then
swiotlb_alloc_coherent() is called as a fallback.

The main part of supporting DMA CMA with swiotlb is that changing
x86_swiotlb_free_coherent() which is .free callback in swiotlb_dma_ops
for dma_free_coherent() so that it can distinguish memory allocated by
dma_generic_alloc_coherent() from one allocated by swiotlb_alloc_coherent()
and release it with dma_generic_free_coherent() which can handle contiguous
memory.  This change requires making is_swiotlb_buffer() global function.

This also needs to change .free callback in the dma_map_ops for amd_gart
and sta2x11, because these dma_ops are also using
dma_generic_alloc_coherent().

Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
---
No change from the previous version

 arch/x86/Kconfig               | 2 +-
 arch/x86/include/asm/swiotlb.h | 7 +++++++
 arch/x86/kernel/amd_gart_64.c  | 2 +-
 arch/x86/kernel/pci-swiotlb.c  | 9 ++++++---
 arch/x86/pci/sta2x11-fixup.c   | 6 ++----
 include/linux/swiotlb.h        | 2 ++
 lib/swiotlb.c                  | 2 +-
 7 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e903c71..b15df8b 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -39,7 +39,7 @@ config X86
 	select ARCH_WANT_OPTIONAL_GPIOLIB
 	select ARCH_WANT_FRAME_POINTERS
 	select HAVE_DMA_ATTRS
-	select HAVE_DMA_CONTIGUOUS if !SWIOTLB
+	select HAVE_DMA_CONTIGUOUS
 	select HAVE_KRETPROBES
 	select HAVE_OPTPROBES
 	select HAVE_KPROBES_ON_FTRACE
diff --git a/arch/x86/include/asm/swiotlb.h b/arch/x86/include/asm/swiotlb.h
index 977f176..ab05d73 100644
--- a/arch/x86/include/asm/swiotlb.h
+++ b/arch/x86/include/asm/swiotlb.h
@@ -29,4 +29,11 @@ static inline void pci_swiotlb_late_init(void)
 
 static inline void dma_mark_clean(void *addr, size_t size) {}
 
+extern void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
+					dma_addr_t *dma_handle, gfp_t flags,
+					struct dma_attrs *attrs);
+extern void x86_swiotlb_free_coherent(struct device *dev, size_t size,
+					void *vaddr, dma_addr_t dma_addr,
+					struct dma_attrs *attrs);
+
 #endif /* _ASM_X86_SWIOTLB_H */
diff --git a/arch/x86/kernel/amd_gart_64.c b/arch/x86/kernel/amd_gart_64.c
index b574b29..8e3842f 100644
--- a/arch/x86/kernel/amd_gart_64.c
+++ b/arch/x86/kernel/amd_gart_64.c
@@ -512,7 +512,7 @@ gart_free_coherent(struct device *dev, size_t size, void *vaddr,
 		   dma_addr_t dma_addr, struct dma_attrs *attrs)
 {
 	gart_unmap_page(dev, dma_addr, size, DMA_BIDIRECTIONAL, NULL);
-	free_pages((unsigned long)vaddr, get_order(size));
+	dma_generic_free_coherent(dev, size, vaddr, dma_addr, attrs);
 }
 
 static int gart_mapping_error(struct device *dev, dma_addr_t dma_addr)
diff --git a/arch/x86/kernel/pci-swiotlb.c b/arch/x86/kernel/pci-swiotlb.c
index 6c483ba..77dd0ad 100644
--- a/arch/x86/kernel/pci-swiotlb.c
+++ b/arch/x86/kernel/pci-swiotlb.c
@@ -14,7 +14,7 @@
 #include <asm/iommu_table.h>
 int swiotlb __read_mostly;
 
-static void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
+void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
 					dma_addr_t *dma_handle, gfp_t flags,
 					struct dma_attrs *attrs)
 {
@@ -28,11 +28,14 @@ static void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
 	return swiotlb_alloc_coherent(hwdev, size, dma_handle, flags);
 }
 
-static void x86_swiotlb_free_coherent(struct device *dev, size_t size,
+void x86_swiotlb_free_coherent(struct device *dev, size_t size,
 				      void *vaddr, dma_addr_t dma_addr,
 				      struct dma_attrs *attrs)
 {
-	swiotlb_free_coherent(dev, size, vaddr, dma_addr);
+	if (is_swiotlb_buffer(dma_to_phys(dev, dma_addr)))
+		swiotlb_free_coherent(dev, size, vaddr, dma_addr);
+	else
+		dma_generic_free_coherent(dev, size, vaddr, dma_addr, attrs);
 }
 
 static struct dma_map_ops swiotlb_dma_ops = {
diff --git a/arch/x86/pci/sta2x11-fixup.c b/arch/x86/pci/sta2x11-fixup.c
index 9d8a509..5ceda85 100644
--- a/arch/x86/pci/sta2x11-fixup.c
+++ b/arch/x86/pci/sta2x11-fixup.c
@@ -173,9 +173,7 @@ static void *sta2x11_swiotlb_alloc_coherent(struct device *dev,
 {
 	void *vaddr;
 
-	vaddr = dma_generic_alloc_coherent(dev, size, dma_handle, flags, attrs);
-	if (!vaddr)
-		vaddr = swiotlb_alloc_coherent(dev, size, dma_handle, flags);
+	vaddr = x86_swiotlb_alloc_coherent(dev, size, dma_handle, flags, attrs);
 	*dma_handle = p2a(*dma_handle, to_pci_dev(dev));
 	return vaddr;
 }
@@ -183,7 +181,7 @@ static void *sta2x11_swiotlb_alloc_coherent(struct device *dev,
 /* We have our own dma_ops: the same as swiotlb but from alloc (above) */
 static struct dma_map_ops sta2x11_dma_ops = {
 	.alloc = sta2x11_swiotlb_alloc_coherent,
-	.free = swiotlb_free_coherent,
+	.free = x86_swiotlb_free_coherent,
 	.map_page = swiotlb_map_page,
 	.unmap_page = swiotlb_unmap_page,
 	.map_sg = swiotlb_map_sg_attrs,
diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index a5ffd32..e7a018e 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -116,4 +116,6 @@ static inline void swiotlb_free(void) { }
 #endif
 
 extern void swiotlb_print_info(void);
+extern int is_swiotlb_buffer(phys_addr_t paddr);
+
 #endif /* __LINUX_SWIOTLB_H */
diff --git a/lib/swiotlb.c b/lib/swiotlb.c
index fe978e0..6e4a798 100644
--- a/lib/swiotlb.c
+++ b/lib/swiotlb.c
@@ -369,7 +369,7 @@ void __init swiotlb_free(void)
 	io_tlb_nslabs = 0;
 }
 
-static int is_swiotlb_buffer(phys_addr_t paddr)
+int is_swiotlb_buffer(phys_addr_t paddr)
 {
 	return paddr >= io_tlb_start && paddr < io_tlb_end;
 }
-- 
1.8.3.2

Re: [PATCH 1/2] x86: enable DMA CMA with swiotlb

[Andi Kleen]

> This also needs to change .free callback in the dma_map_ops for amd_gart
> and sta2x11, because these dma_ops are also using
> dma_generic_alloc_coherent().

Looks good to me from a quick read.

-Andi

Re: [PATCH 1/2] x86: enable DMA CMA with swiotlb

[Konrad Rzeszutek Wilk]

On Fri, Nov 29, 2013 at 11:51:45PM +0900, Akinobu Mita wrote:
> The DMA Contiguous Memory Allocator support on x86 is disabled when
> swiotlb config option is enabled.  So DMA CMA is always disabled on
> x86_64 because swiotlb is always enabled.  This attempts to support
> for DMA CMA with enabling swiotlb config option.
> 
> The contiguous memory allocator on x86 is integrated in the function
> dma_generic_alloc_coherent() which is .alloc callback in nommu_dma_ops
> for dma_alloc_coherent().
> 
> x86_swiotlb_alloc_coherent() which is .alloc callback in swiotlb_dma_ops
> tries to allocate with dma_generic_alloc_coherent() firstly and then
> swiotlb_alloc_coherent() is called as a fallback.
> 
> The main part of supporting DMA CMA with swiotlb is that changing
> x86_swiotlb_free_coherent() which is .free callback in swiotlb_dma_ops
> for dma_free_coherent() so that it can distinguish memory allocated by
> dma_generic_alloc_coherent() from one allocated by swiotlb_alloc_coherent()
> and release it with dma_generic_free_coherent() which can handle contiguous
> memory.  This change requires making is_swiotlb_buffer() global function.
> 
> This also needs to change .free callback in the dma_map_ops for amd_gart
> and sta2x11, because these dma_ops are also using
> dma_generic_alloc_coherent().
> 

Before I queue it up and start testing it - Peter were your
concerns answered?

Thanks!
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: x86@kernel.org
> Cc: Andi Kleen <andi@firstfloor.org>
> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
> ---
> No change from the previous version
> 
>  arch/x86/Kconfig               | 2 +-
>  arch/x86/include/asm/swiotlb.h | 7 +++++++
>  arch/x86/kernel/amd_gart_64.c  | 2 +-
>  arch/x86/kernel/pci-swiotlb.c  | 9 ++++++---
>  arch/x86/pci/sta2x11-fixup.c   | 6 ++----
>  include/linux/swiotlb.h        | 2 ++
>  lib/swiotlb.c                  | 2 +-
>  7 files changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index e903c71..b15df8b 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -39,7 +39,7 @@ config X86
>  	select ARCH_WANT_OPTIONAL_GPIOLIB
>  	select ARCH_WANT_FRAME_POINTERS
>  	select HAVE_DMA_ATTRS
> -	select HAVE_DMA_CONTIGUOUS if !SWIOTLB
> +	select HAVE_DMA_CONTIGUOUS
>  	select HAVE_KRETPROBES
>  	select HAVE_OPTPROBES
>  	select HAVE_KPROBES_ON_FTRACE
> diff --git a/arch/x86/include/asm/swiotlb.h b/arch/x86/include/asm/swiotlb.h
> index 977f176..ab05d73 100644
> --- a/arch/x86/include/asm/swiotlb.h
> +++ b/arch/x86/include/asm/swiotlb.h
> @@ -29,4 +29,11 @@ static inline void pci_swiotlb_late_init(void)
>  
>  static inline void dma_mark_clean(void *addr, size_t size) {}
>  
> +extern void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
> +					dma_addr_t *dma_handle, gfp_t flags,
> +					struct dma_attrs *attrs);
> +extern void x86_swiotlb_free_coherent(struct device *dev, size_t size,
> +					void *vaddr, dma_addr_t dma_addr,
> +					struct dma_attrs *attrs);
> +
>  #endif /* _ASM_X86_SWIOTLB_H */
> diff --git a/arch/x86/kernel/amd_gart_64.c b/arch/x86/kernel/amd_gart_64.c
> index b574b29..8e3842f 100644
> --- a/arch/x86/kernel/amd_gart_64.c
> +++ b/arch/x86/kernel/amd_gart_64.c
> @@ -512,7 +512,7 @@ gart_free_coherent(struct device *dev, size_t size, void *vaddr,
>  		   dma_addr_t dma_addr, struct dma_attrs *attrs)
>  {
>  	gart_unmap_page(dev, dma_addr, size, DMA_BIDIRECTIONAL, NULL);
> -	free_pages((unsigned long)vaddr, get_order(size));
> +	dma_generic_free_coherent(dev, size, vaddr, dma_addr, attrs);
>  }
>  
>  static int gart_mapping_error(struct device *dev, dma_addr_t dma_addr)
> diff --git a/arch/x86/kernel/pci-swiotlb.c b/arch/x86/kernel/pci-swiotlb.c
> index 6c483ba..77dd0ad 100644
> --- a/arch/x86/kernel/pci-swiotlb.c
> +++ b/arch/x86/kernel/pci-swiotlb.c
> @@ -14,7 +14,7 @@
>  #include <asm/iommu_table.h>
>  int swiotlb __read_mostly;
>  
> -static void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
> +void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
>  					dma_addr_t *dma_handle, gfp_t flags,
>  					struct dma_attrs *attrs)
>  {
> @@ -28,11 +28,14 @@ static void *x86_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
>  	return swiotlb_alloc_coherent(hwdev, size, dma_handle, flags);
>  }
>  
> -static void x86_swiotlb_free_coherent(struct device *dev, size_t size,
> +void x86_swiotlb_free_coherent(struct device *dev, size_t size,
>  				      void *vaddr, dma_addr_t dma_addr,
>  				      struct dma_attrs *attrs)
>  {
> -	swiotlb_free_coherent(dev, size, vaddr, dma_addr);
> +	if (is_swiotlb_buffer(dma_to_phys(dev, dma_addr)))
> +		swiotlb_free_coherent(dev, size, vaddr, dma_addr);
> +	else
> +		dma_generic_free_coherent(dev, size, vaddr, dma_addr, attrs);
>  }
>  
>  static struct dma_map_ops swiotlb_dma_ops = {
> diff --git a/arch/x86/pci/sta2x11-fixup.c b/arch/x86/pci/sta2x11-fixup.c
> index 9d8a509..5ceda85 100644
> --- a/arch/x86/pci/sta2x11-fixup.c
> +++ b/arch/x86/pci/sta2x11-fixup.c
> @@ -173,9 +173,7 @@ static void *sta2x11_swiotlb_alloc_coherent(struct device *dev,
>  {
>  	void *vaddr;
>  
> -	vaddr = dma_generic_alloc_coherent(dev, size, dma_handle, flags, attrs);
> -	if (!vaddr)
> -		vaddr = swiotlb_alloc_coherent(dev, size, dma_handle, flags);
> +	vaddr = x86_swiotlb_alloc_coherent(dev, size, dma_handle, flags, attrs);
>  	*dma_handle = p2a(*dma_handle, to_pci_dev(dev));
>  	return vaddr;
>  }
> @@ -183,7 +181,7 @@ static void *sta2x11_swiotlb_alloc_coherent(struct device *dev,
>  /* We have our own dma_ops: the same as swiotlb but from alloc (above) */
>  static struct dma_map_ops sta2x11_dma_ops = {
>  	.alloc = sta2x11_swiotlb_alloc_coherent,
> -	.free = swiotlb_free_coherent,
> +	.free = x86_swiotlb_free_coherent,
>  	.map_page = swiotlb_map_page,
>  	.unmap_page = swiotlb_unmap_page,
>  	.map_sg = swiotlb_map_sg_attrs,
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index a5ffd32..e7a018e 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -116,4 +116,6 @@ static inline void swiotlb_free(void) { }
>  #endif
>  
>  extern void swiotlb_print_info(void);
> +extern int is_swiotlb_buffer(phys_addr_t paddr);
> +
>  #endif /* __LINUX_SWIOTLB_H */
> diff --git a/lib/swiotlb.c b/lib/swiotlb.c
> index fe978e0..6e4a798 100644
> --- a/lib/swiotlb.c
> +++ b/lib/swiotlb.c
> @@ -369,7 +369,7 @@ void __init swiotlb_free(void)
>  	io_tlb_nslabs = 0;
>  }
>  
> -static int is_swiotlb_buffer(phys_addr_t paddr)
> +int is_swiotlb_buffer(phys_addr_t paddr)
>  {
>  	return paddr >= io_tlb_start && paddr < io_tlb_end;
>  }
> -- 
> 1.8.3.2
> 

[PATCH 2/2] intel-iommu: integrate DMA CMA

[Akinobu Mita]

This adds support for the DMA Contiguous Memory Allocator for intel-iommu.
This change enables dma_alloc_coherent() to allocate big contiguous
memory.

It is achieved in the same way as nommu_dma_ops currently does, i.e.
trying to allocate memory by dma_alloc_from_contiguous() and alloc_pages()
is used as a fallback.

Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: iommu@lists.linux-foundation.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
---
 drivers/iommu/intel-iommu.c | 32 ++++++++++++++++++++++++--------
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index fd426ca..172c2b0 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -3004,7 +3004,7 @@ static void *intel_alloc_coherent(struct device *hwdev, size_t size,
 				  dma_addr_t *dma_handle, gfp_t flags,
 				  struct dma_attrs *attrs)
 {
-	void *vaddr;
+	struct page *page = NULL;
 	int order;
 
 	size = PAGE_ALIGN(size);
@@ -3019,17 +3019,31 @@ static void *intel_alloc_coherent(struct device *hwdev, size_t size,
 			flags |= GFP_DMA32;
 	}
 
-	vaddr = (void *)__get_free_pages(flags, order);
-	if (!vaddr)
+	if (!(flags & GFP_ATOMIC)) {
+		unsigned int count = size >> PAGE_SHIFT;
+
+		page = dma_alloc_from_contiguous(hwdev, count, order);
+		if (page && iommu_no_mapping(hwdev) &&
+		    page_to_phys(page) + size > hwdev->coherent_dma_mask) {
+			dma_release_from_contiguous(hwdev, page, count);
+			page = NULL;
+		}
+	}
+
+	if (!page)
+		page = alloc_pages(flags, order);
+	if (!page)
 		return NULL;
-	memset(vaddr, 0, size);
+	memset(page_address(page), 0, size);
 
-	*dma_handle = __intel_map_single(hwdev, virt_to_bus(vaddr), size,
+	*dma_handle = __intel_map_single(hwdev, page_to_phys(page), size,
 					 DMA_BIDIRECTIONAL,
 					 hwdev->coherent_dma_mask);
 	if (*dma_handle)
-		return vaddr;
-	free_pages((unsigned long)vaddr, order);
+		return page_address(page);
+	if (!dma_release_from_contiguous(hwdev, page, size >> PAGE_SHIFT))
+		__free_pages(page, order);
+
 	return NULL;
 }
 
@@ -3037,12 +3051,14 @@ static void intel_free_coherent(struct device *hwdev, size_t size, void *vaddr,
 				dma_addr_t dma_handle, struct dma_attrs *attrs)
 {
 	int order;
+	struct page *page = virt_to_page(vaddr);
 
 	size = PAGE_ALIGN(size);
 	order = get_order(size);
 
 	intel_unmap_page(hwdev, dma_handle, size, DMA_BIDIRECTIONAL, NULL);
-	free_pages((unsigned long)vaddr, order);
+	if (!dma_release_from_contiguous(hwdev, page, size >> PAGE_SHIFT))
+		__free_pages(page, order);
 }
 
 static void intel_unmap_sg(struct device *hwdev, struct scatterlist *sglist,
-- 
1.8.3.2

Re: [PATCH 0/2] extend support for CMA CMA on x86

[Andi Kleen]

On Fri, Nov 29, 2013 at 11:51:44PM +0900, Akinobu Mita wrote:
> This patch set extends support for the DMA Contiguous Memory Allocator
> on x86.  Currently it is only supported on pci-nommu.  So this aims at
> enabling it on swiotlb and intel-iommu, too.
> 
> Regardless of which dma mapping implementation is actually used in the
> system, I would like to allocate big contiguous memory with
> dma_alloc_coherent() and tell the base address to the device that
> requires it.  This is why I need this change.

I think it's generally the right direction. CMA is much better
than the DMA zone (e.g. it can actually (mostly) share memory,
while lower zone protection keeps the DMA zone free)
Also it works for more than 16MB / 64MB.

So i hope longer term we can just kill GFP_DMA and replace
it with some CMA variant everywhere.

There are also other potential users that could make use of it,
but they need to interoperate with swiotlb, so something liike
this is needed.

-Andi