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Message-ID: <37609731-5539-b906-aa94-2ef0242795ac@arm.com>
Date: Mon, 14 Oct 2019 11:29:24 +0100
From: Robin Murphy <robin.murphy@....com>
To: David Gibson <david@...son.dropbear.id.au>,
Ram Pai <linuxram@...ibm.com>
Cc: linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
linuxppc-dev@...ts.ozlabs.org,
virtualization@...ts.linux-foundation.org,
benh@...nel.crashing.org, mpe@...erman.id.au, paulus@...abs.org,
mdroth@...ux.vnet.ibm.com, aik@...ux.ibm.com, paul.burton@...s.com,
b.zolnierkie@...sung.com, m.szyprowski@...sung.com, hch@....de,
jasowang@...hat.com, andmike@...ibm.com, sukadev@...ux.vnet.ibm.com
Subject: Re: [PATCH 1/2] dma-mapping: Add dma_addr_is_phys_addr()
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@...ux.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@...nel.crashing.org>
>> cc: David Gibson <david@...son.dropbear.id.au>
>> cc: Michael Ellerman <mpe@...erman.id.au>
>> cc: Paul Mackerras <paulus@...abs.org>
>> cc: Michael Roth <mdroth@...ux.vnet.ibm.com>
>> cc: Alexey Kardashevskiy <aik@...ux.ibm.com>
>> cc: Paul Burton <paul.burton@...s.com>
>> cc: Robin Murphy <robin.murphy@....com>
>> cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
>> cc: Marek Szyprowski <m.szyprowski@...sung.com>
>> cc: Christoph Hellwig <hch@....de>
>> Suggested-by: Michael S. Tsirkin <mst@...hat.com>
>> Signed-off-by: Ram Pai <linuxram@...ibm.com>
>> Signed-off-by: Thiago Jung Bauermann <bauerman@...ux.ibm.com>
>
> The change itself looks ok, so
>
> Reviewed-by: David Gibson <david@...son.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
>>
>
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