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    SubjectRe: [PATCH v7 01/21] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL



    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?

    Logan

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