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

[Logan Gunthorpe <logang@deltatee.com>]

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