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

From: David Gibson <[email-masked]>
Date: Mon, 14 Oct 2019 15:51:39 +1100

On Fri, Oct 11, 2019 at 06:25:18PM -0700, Ram Pai wrote:
> From: Thiago Jung Bauermann <[email-masked]>
> 
> 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 <[email-masked]>
> cc: David Gibson <[email-masked]>
> cc: Michael Ellerman <[email-masked]>
> cc: Paul Mackerras <[email-masked]>
> cc: Michael Roth <[email-masked]>
> cc: Alexey Kardashevskiy <[email-masked]>
> cc: Paul Burton <[email-masked]>
> cc: Robin Murphy <[email-masked]>
> cc: Bartlomiej Zolnierkiewicz <[email-masked]>
> cc: Marek Szyprowski <[email-masked]>
> cc: Christoph Hellwig <[email-masked]>
> Suggested-by: Michael S. Tsirkin <[email-masked]>
> Signed-off-by: Ram Pai <[email-masked]>
> Signed-off-by: Thiago Jung Bauermann <[email-masked]>

The change itself looks ok, so

Reviewed-by: David Gibson <[email-masked]>

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.

> ---
>  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
>  

-- 
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