linux-kernel - Re: [PATCH v3 01/20] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 28 Sep 2021 15:32:19 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>
Subject: Re: [PATCH v3 01/20] lib/scatterlist: add flag for indicating P2PDMA
 segments in an SGL

On Thu, Sep 16, 2021 at 05:40:41PM -0600, 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 P2PDMA.
> 
> Using this bit requires adding an additional dependency on CONFIG_64BIT to
> CONFIG_PCI_P2PDMA. 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@...tatee.com>
> Reviewed-by: John Hubbard <jhubbard@...dia.com>
>  drivers/pci/Kconfig         |  2 +-
>  include/linux/scatterlist.h | 50 ++++++++++++++++++++++++++++++++++---
>  2 files changed, 47 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
> index 0c473d75e625..90b4bddb3300 100644
> +++ b/drivers/pci/Kconfig
> @@ -163,7 +163,7 @@ config PCI_PASID
>  
>  config PCI_P2PDMA
>  	bool "PCI peer-to-peer transfer support"
> -	depends on ZONE_DEVICE
> +	depends on ZONE_DEVICE && 64BIT

Perhaps a comment to explain what the 64bit is doing?

>  	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 266754a55327..e62b1cf6386f 100644
> +++ b/include/linux/scatterlist.h
> @@ -64,6 +64,21 @@ 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 PCI
> + * bus address when doing P2PDMA.
> + * Note: CONFIG_PCI_P2PDMA depends on CONFIG_64BIT because of this.
> + */
> +
> +#ifdef CONFIG_PCI_P2PDMA
> +#define SG_DMA_PCI_P2PDMA	0x04UL

Add a 
	static_assert(__alignof__(void *) == 8);

?

> +#else
> +#define SG_DMA_PCI_P2PDMA	0x00UL
> +#endif
> +
> +#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END | SG_DMA_PCI_P2PDMA)
> +
>  /*
>   * 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.
> @@ -72,7 +87,9 @@ struct sg_append_table {
>  #define sg_is_chain(sg)		((sg)->page_link & SG_CHAIN)
>  #define sg_is_last(sg)		((sg)->page_link & SG_END)
>  #define sg_chain_ptr(sg)	\
> -	((struct scatterlist *) ((sg)->page_link & ~(SG_CHAIN | SG_END)))
> +	((struct scatterlist *)((sg)->page_link & ~SG_PAGE_LINK_MASK))
> +
> +#define sg_is_dma_pci_p2pdma(sg) ((sg)->page_link & SG_DMA_PCI_P2PDMA)

I've been encouraging people to use static inlines more..

static inline unsigned int __sg_flags(struct scatterlist *sg)
{
	return sg->page_link & SG_PAGE_LINK_MASK;
}
static inline bool sg_is_chain(struct scatterlist *sg)
{
	return __sg_flags(sg) & SG_CHAIN;
}
static inline bool sg_is_last(struct scatterlist *sg)
{
	return __sg_flags(sg) & SG_END;
}
static inline bool sg_is_dma_pci_p2pdma(struct scatterlist *sg)
{
	return __sg_flags(sg) & SG_DMA_PCI_P2PDMA;
}

>  /**
>   * sg_assign_page - Assign a given page to an SG entry
> @@ -86,13 +103,13 @@ struct sg_append_table {
>   **/
>  static inline void sg_assign_page(struct scatterlist *sg, struct page *page)
>  {
> -	unsigned long page_link = sg->page_link & (SG_CHAIN | SG_END);
> +	unsigned long page_link = sg->page_link & SG_PAGE_LINK_MASK;

I think this should just be '& SG_END', sg_assign_page() doesn't look
like it should ever be used on a sg_chain entry, so this is just
trying to preserve the end stamp.

Anyway, this looks OK

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason