Messages in this thread | | | Date | Wed, 29 Jun 2022 19:02:33 +0100 | Subject | Re: [PATCH v7 01/21] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL | From | Robin Murphy <> |
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On 2022-06-29 16:39, Logan Gunthorpe wrote: > > > > On 2022-06-29 03:05, Robin Murphy wrote: >> On 2022-06-15 17:12, Logan Gunthorpe wrote: >>> Make use of the third free LSB in scatterlist's page_link on 64bit >>> systems. >>> >>> The extra bit will be used by dma_[un]map_sg_p2pdma() to determine when a >>> given SGL segments dma_address points to a PCI bus address. >>> dma_unmap_sg_p2pdma() will need to perform different cleanup when a >>> segment is marked as a bus address. >>> >>> The new bit will only be used when CONFIG_PCI_P2PDMA is set; this means >>> PCI P2PDMA will require CONFIG_64BIT. This should be acceptable as the >>> majority of P2PDMA use cases are restricted to newer root complexes and >>> roughly require the extra address space for memory BARs used in the >>> transactions. >>> >>> Signed-off-by: Logan Gunthorpe <logang@deltatee.com> >>> Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com> >>> --- >>> drivers/pci/Kconfig | 5 +++++ >>> include/linux/scatterlist.h | 44 ++++++++++++++++++++++++++++++++++++- >>> 2 files changed, 48 insertions(+), 1 deletion(-) >>> >>> diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig >>> index 133c73207782..5cc7cba1941f 100644 >>> --- a/drivers/pci/Kconfig >>> +++ b/drivers/pci/Kconfig >>> @@ -164,6 +164,11 @@ config PCI_PASID >>> config PCI_P2PDMA >>> bool "PCI peer-to-peer transfer support" >>> depends on ZONE_DEVICE >>> + # >>> + # The need for the scatterlist DMA bus address flag means PCI P2PDMA >>> + # requires 64bit >>> + # >>> + depends on 64BIT >>> select GENERIC_ALLOCATOR >>> help >>> Enableѕ drivers to do PCI peer-to-peer transactions to and from >>> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h >>> index 7ff9d6386c12..6561ca8aead8 100644 >>> --- a/include/linux/scatterlist.h >>> +++ b/include/linux/scatterlist.h >>> @@ -64,12 +64,24 @@ struct sg_append_table { >>> #define SG_CHAIN 0x01UL >>> #define SG_END 0x02UL >>> +/* >>> + * bit 2 is the third free bit in the page_link on 64bit systems which >>> + * is used by dma_unmap_sg() to determine if the dma_address is a >>> + * bus address when doing P2PDMA. >>> + */ >>> +#ifdef CONFIG_PCI_P2PDMA >>> +#define SG_DMA_BUS_ADDRESS 0x04UL >>> +static_assert(__alignof__(struct page) >= 8); >>> +#else >>> +#define SG_DMA_BUS_ADDRESS 0x00UL >>> +#endif >>> + >>> /* >>> * We overload the LSB of the page pointer to indicate whether it's >>> * a valid sg entry, or whether it points to the start of a new >>> scatterlist. >>> * Those low bits are there for everyone! (thanks mason :-) >>> */ >>> -#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END) >>> +#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END | SG_DMA_BUS_ADDRESS) >>> static inline unsigned int __sg_flags(struct scatterlist *sg) >>> { >>> @@ -91,6 +103,11 @@ static inline bool sg_is_last(struct scatterlist *sg) >>> return __sg_flags(sg) & SG_END; >>> } >>> +static inline bool sg_is_dma_bus_address(struct scatterlist *sg) >>> +{ >>> + return __sg_flags(sg) & SG_DMA_BUS_ADDRESS; >>> +} >>> + >>> /** >>> * sg_assign_page - Assign a given page to an SG entry >>> * @sg: SG entry >>> @@ -245,6 +262,31 @@ static inline void sg_unmark_end(struct >>> scatterlist *sg) >>> sg->page_link &= ~SG_END; >>> } >>> +/** >>> + * sg_dma_mark_bus address - Mark the scatterlist entry as a bus address >>> + * @sg: SG entryScatterlist >> >> entryScatterlist? >> >>> + * >>> + * Description: >>> + * Marks the passed in sg entry to indicate that the dma_address is >>> + * a bus address and doesn't need to be unmapped. >>> + **/ >>> +static inline void sg_dma_mark_bus_address(struct scatterlist *sg) >>> +{ >>> + sg->page_link |= SG_DMA_BUS_ADDRESS; >>> +} >>> + >>> +/** >>> + * sg_unmark_pci_p2pdma - Unmark the scatterlist entry as a bus address >>> + * @sg: SG entryScatterlist >>> + * >>> + * Description: >>> + * Clears the bus address mark. >>> + **/ >>> +static inline void sg_dma_unmark_bus_address(struct scatterlist *sg) >>> +{ >>> + sg->page_link &= ~SG_DMA_BUS_ADDRESS; >>> +} >> >> Does this serve any useful purpose? If a page is determined to be device >> memory, it's not going to suddenly stop being device memory, and if the >> underlying sg is recycled to point elsewhere then sg_assign_page() will >> still (correctly) clear this flag anyway. Trying to reason about this >> beyond superficial API symmetry - i.e. why exactly would a caller need >> to call it, and what would the implications be of failing to do so - >> seems to lead straight to confusion. >> >> In fact I'd be inclined to have sg_assign_page() be responsible for >> setting the flag automatically as well, and thus not need >> sg_dma_mark_bus_address() either, however I can see the argument for >> doing it this way round to not entangle the APIs too much, so I don't >> have any great objection to that. > > Yes, I think you misunderstand what this is for. The SG_DMA_BUS_ADDDRESS > flag doesn't mark the segment for the page, but for the dma address. It > cannot be set in sg_assign_page() seeing it's not a property of the page > but a property of the dma_address in the sgl. > > It's not meant for use by regular SG users, it's only meant for use > inside DMA mapping implementations. The purpose is to know whether a > given dma_address in the SGL is a bus address or regular memory because > the two different types must be unmapped differently. We can't rely on > the page because, as you know, many dma_map_sg() the dma_address entry > in the sgl does not map to the same memory as the page. Or to put it > another way: is_pci_p2pdma_page(sg->page) does not imply that > sg->dma_address points to a bus address. > > Does that make sense?
Ah, you're quite right, in trying to take in the whole series at once first thing in the morning I did fail to properly grasp that detail, so indeed the sg_assign_page() thing couldn't possibly work, but as I said that's fine anyway. I still think the lifecycle management is a bit off though - equivalently, a bus address doesn't stop being a bus address, so it would seem appropriate to update this flag appropriately whenever sg_dma_address() is assigned to, and not when it isn't.
Thanks, Robin.
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