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

[Robin Murphy <robin.murphy@arm.com>]

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@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 <benh@kernel.crashing.org>
>> cc: David Gibson <david@gibson.dropbear.id.au>
>> cc: Michael Ellerman <mpe@ellerman.id.au>
>> cc: Paul Mackerras <paulus@ozlabs.org>
>> cc: Michael Roth <mdroth@linux.vnet.ibm.com>
>> cc: Alexey Kardashevskiy <aik@linux.ibm.com>
>> cc: Paul Burton <paul.burton@mips.com>
>> cc: Robin Murphy <robin.murphy@arm.com>
>> cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
>> cc: Marek Szyprowski <m.szyprowski@samsung.com>
>> cc: Christoph Hellwig <hch@lst.de>
>> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
>> Signed-off-by: Ram Pai <linuxram@us.ibm.com>
>> Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
> 
> The change itself looks ok, so
> 
> Reviewed-by: David Gibson <david@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
>>   
>