Re: [PATCH 08/16] PCI/P2PDMA: Introduce helpers for dma_map_sg implementations

From: John Hubbard <jhubbard@nvidia.com>
To: Logan Gunthorpe <logang@deltatee.com>,
	<linux-kernel@vger.kernel.org>, <linux-nvme@lists.infradead.org>,
	<linux-block@vger.kernel.org>, <linux-pci@vger.kernel.org>,
	<linux-mm@kvack.org>, <iommu@lists.linux-foundation.org>
Cc: "Stephen Bates" <sbates@raithlin.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Christian König" <christian.koenig@amd.com>,
	"Don Dutile" <ddutile@redhat.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>,
	"Jakowski Andrzej" <andrzej.jakowski@intel.com>,
	"Minturn Dave B" <dave.b.minturn@intel.com>,
	"Jason Ekstrand" <jason@jlekstrand.net>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Xiong Jianxin" <jianxin.xiong@intel.com>,
	"Bjorn Helgaas" <helgaas@kernel.org>,
	"Ira Weiny" <ira.weiny@intel.com>,
	"Robin Murphy" <robin.murphy@arm.com>
Subject: Re: [PATCH 08/16] PCI/P2PDMA: Introduce helpers for dma_map_sg implementations
Date: Sun, 2 May 2021 17:50:39 -0700	[thread overview]
Message-ID: <e27d35f8-5c3a-39e3-9845-6d2bf15cc8b3@nvidia.com> (raw)
In-Reply-To: <20210408170123.8788-9-logang@deltatee.com>

On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
> Add pci_p2pdma_map_segment() as a helper for simple dma_map_sg()
> implementations. It takes an scatterlist segment that must point to a
> pci_p2pdma struct page and will map it if the mapping requires a bus
> address.
> 
> The return value indicates whether the mapping required a bus address
> or whether the caller still needs to map the segment normally. If the
> segment should not be mapped, -EREMOTEIO is returned.
> 
> This helper uses a state structure to track the changes to the
> pgmap across calls and avoid needing to lookup into the xarray for
> every page.
> 

OK, coming back to this patch, after seeing how it is used later in
the series...

> Also add pci_p2pdma_map_bus_segment() which is useful for IOMMU
> dma_map_sg() implementations where the sg segment containing the page
> differs from the sg segment containing the DMA address.
> 
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
>   drivers/pci/p2pdma.c       | 65 ++++++++++++++++++++++++++++++++++++++
>   include/linux/pci-p2pdma.h | 21 ++++++++++++
>   2 files changed, 86 insertions(+)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 38c93f57a941..44ad7664e875 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -923,6 +923,71 @@ void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>   }
>   EXPORT_SYMBOL_GPL(pci_p2pdma_unmap_sg_attrs);
>   
> +/**
> + * pci_p2pdma_map_segment - map an sg segment determining the mapping type
> + * @state: State structure that should be declared on the stack outside of
> + *	the for_each_sg() loop and initialized to zero.

Silly fine point for the docs here: it doesn't actually have to be on
the stack, so I don't think you need to write that constraint in the
documentation. It just has be be somehow allocated and zeroed.

> + * @dev: DMA device that's doing the mapping operation
> + * @sg: scatterlist segment to map
> + * @attrs: dma mapping attributes
> + *
> + * This is a helper to be used by non-iommu dma_map_sg() implementations where
> + * the sg segment is the same for the page_link and the dma_address.
> + *
> + * Attempt to map a single segment in an SGL with the PCI bus address.
> + * The segment must point to a PCI P2PDMA page and thus must be
> + * wrapped in a is_pci_p2pdma_page(sg_page(sg)) check.

Should this be backed up with actual checks in the function, that
the prerequisites are met?

> + *
> + * Returns 1 if the segment was mapped, 0 if the segment should be mapped
> + * directly (or through the IOMMU) and -EREMOTEIO if the segment should not
> + * be mapped at all.
> + */
> +int pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state,
> +			   struct device *dev, struct scatterlist *sg,
> +			   unsigned long dma_attrs)
> +{
> +	if (state->pgmap != sg_page(sg)->pgmap) {
> +		state->pgmap = sg_page(sg)->pgmap;
> +		state->map = pci_p2pdma_map_type(state->pgmap, dev, dma_attrs);
> +		state->bus_off = to_p2p_pgmap(state->pgmap)->bus_offset;
> +	}

I'll quote myself from patch 9, because I had a comment there that actually
was meant for this patch:

Is it worth putting this stuff on the caller's stack? I mean, is there a
noticeable performance improvement from caching the state? Because if
it's invisible, then simplicity is better. I suspect you're right, and
that it *is* worth it, but it's good to know for real.

> +
> +	switch (state->map) {
> +	case PCI_P2PDMA_MAP_BUS_ADDR:
> +		sg->dma_address = sg_phys(sg) + state->bus_off;
> +		sg_dma_len(sg) = sg->length;
> +		sg_mark_pci_p2pdma(sg);
> +		return 1;
> +	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> +		return 0;
> +	default:
> +		return -EREMOTEIO;
> +	}
> +}
> +
> +/**
> + * pci_p2pdma_map_bus_segment - map an sg segment pre determined to
> + *	be mapped with PCI_P2PDMA_MAP_BUS_ADDR

Or:

  * pci_p2pdma_map_bus_segment - map an SG segment that is already known
  * to be mapped with PCI_P2PDMA_MAP_BUS_ADDR

Also, should that prerequisite be backed up with checks in the function?

> + * @pg_sg: scatterlist segment with the page to map
> + * @dma_sg: scatterlist segment to assign a dma address to
> + *
> + * This is a helper for iommu dma_map_sg() implementations when the
> + * segment for the dma address differs from the segment containing the
> + * source page.
> + *
> + * pci_p2pdma_map_type() must have already been called on the pg_sg and
> + * returned PCI_P2PDMA_MAP_BUS_ADDR.

Another prerequisite, so same question: do you think that the code should
also check that this prerequisite is met?

thanks,
-- 
John Hubbard
NVIDIA

> + */
> +void pci_p2pdma_map_bus_segment(struct scatterlist *pg_sg,
> +				struct scatterlist *dma_sg)
> +{
> +	struct pci_p2pdma_pagemap *pgmap = to_p2p_pgmap(sg_page(pg_sg)->pgmap);
> +
> +	dma_sg->dma_address = sg_phys(pg_sg) + pgmap->bus_offset;
> +	sg_dma_len(dma_sg) = pg_sg->length;
> +	sg_mark_pci_p2pdma(dma_sg);
> +}
> +
>   /**
>    * pci_p2pdma_enable_store - parse a configfs/sysfs attribute store
>    *		to enable p2pdma
> diff --git a/include/linux/pci-p2pdma.h b/include/linux/pci-p2pdma.h
> index a06072ac3a52..49e7679403cf 100644
> --- a/include/linux/pci-p2pdma.h
> +++ b/include/linux/pci-p2pdma.h
> @@ -13,6 +13,12 @@
>   
>   #include <linux/pci.h>
>   
> +struct pci_p2pdma_map_state {
> +	struct dev_pagemap *pgmap;
> +	int map;
> +	u64 bus_off;
> +};
> +
>   struct block_device;
>   struct scatterlist;
>   
> @@ -43,6 +49,11 @@ int pci_p2pdma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
>   		int nents, enum dma_data_direction dir, unsigned long attrs);
>   void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>   		int nents, enum dma_data_direction dir, unsigned long attrs);
> +int pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state,
> +		struct device *dev, struct scatterlist *sg,
> +		unsigned long dma_attrs);
> +void pci_p2pdma_map_bus_segment(struct scatterlist *pg_sg,
> +				struct scatterlist *dma_sg);
>   int pci_p2pdma_enable_store(const char *page, struct pci_dev **p2p_dev,
>   			    bool *use_p2pdma);
>   ssize_t pci_p2pdma_enable_show(char *page, struct pci_dev *p2p_dev,
> @@ -109,6 +120,16 @@ static inline void pci_p2pdma_unmap_sg_attrs(struct device *dev,
>   		unsigned long attrs)
>   {
>   }
> +static inline int pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state,
> +		struct device *dev, struct scatterlist *sg,
> +		unsigned long dma_attrs)
> +{
> +	return 0;
> +}
> +static inline void pci_p2pdma_map_bus_segment(struct scatterlist *pg_sg,
> +					      struct scatterlist *dma_sg)
> +{
> +}
>   static inline int pci_p2pdma_enable_store(const char *page,
>   		struct pci_dev **p2p_dev, bool *use_p2pdma)
>   {
> 