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

From: Logan Gunthorpe <logang@deltatee.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: 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,
	"Stephen Bates" <sbates@raithlin.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Christian König" <christian.koenig@amd.com>,
	"John Hubbard" <jhubbard@nvidia.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>,
	"Martin Oliveira" <martin.oliveira@eideticom.com>,
	"Chaitanya Kulkarni" <ckulkarnilinux@gmail.com>
Subject: Re: [PATCH v3 01/20] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL
Date: Wed, 29 Sep 2021 15:15:34 -0600	[thread overview]
Message-ID: <3f3abc6b-0a80-dbb6-f9dc-8a4cc33c975c@deltatee.com> (raw)
In-Reply-To: <20210928183219.GJ3544071@ziepe.ca>

On 2021-09-28 12:32 p.m., Jason Gunthorpe wrote:
> On Thu, Sep 16, 2021 at 05:40:41PM -0600, Logan Gunthorpe wrote:
>>  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?

Added.

>>  	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);
> 
> ?

Good idea. Though, I think your line isn't quite correct. I've added:

static_assert(__alignof__(struct page) >= 8);

>> +#define sg_is_dma_pci_p2pdma(sg) ((sg)->page_link & SG_DMA_PCI_P2PDMA)
> 
> I've been encouraging people to use static inlines more..

I also prefer static inlines, but I usually follow the style of the code
I'm changing. In any case, I've changed to static inlines similar to
your example.

>>  /**
>>   * 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.

Perhaps, but I'm not comfortable making that change in this patch or
series. Though, I've reverted this specific change in my patch so
sg_assign_page() will clear SG_DMA_PCI_P2PDMA.

Logan