linux-kernel - Re: [PATCH v1 04/10] PCI/P2PDMA: Refactor to separate core P2P functionality from memory allocation

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Message-ID: <20250806154214.1c2618e8.alex.williamson@redhat.com>
Date: Wed, 6 Aug 2025 15:42:14 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Leon Romanovsky <leonro@...dia.com>, Jason Gunthorpe <jgg@...dia.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Bjorn Helgaas
 <bhelgaas@...gle.com>, Christian König
 <christian.koenig@....com>, Christoph Hellwig <hch@....de>,
 dri-devel@...ts.freedesktop.org, iommu@...ts.linux.dev, Jens Axboe
 <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
 linaro-mm-sig@...ts.linaro.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
 linux-mm@...ck.org, linux-pci@...r.kernel.org, Logan Gunthorpe
 <logang@...tatee.com>, Marek Szyprowski <m.szyprowski@...sung.com>, Robin
 Murphy <robin.murphy@....com>, Sumit Semwal <sumit.semwal@...aro.org>,
 Vivek Kasireddy <vivek.kasireddy@...el.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH v1 04/10] PCI/P2PDMA: Refactor to separate core P2P
 functionality from memory allocation

On Mon,  4 Aug 2025 16:00:39 +0300
Leon Romanovsky <leon@...nel.org> wrote:

> From: Leon Romanovsky <leonro@...dia.com>
> 
> Refactor the PCI P2PDMA subsystem to separate the core peer-to-peer DMA
> functionality from the optional memory allocation layer. This creates a
> two-tier architecture:
> 
> The core layer provides P2P mapping functionality for physical addresses
> based on PCI device MMIO BARs and integrates with the DMA API for
> mapping operations. This layer is required for all P2PDMA users.
> 
> The optional upper layer provides memory allocation capabilities
> including gen_pool allocator, struct page support, and sysfs interface
> for user space access.
> 
> This separation allows subsystems like VFIO to use only the core P2P
> mapping functionality without the overhead of memory allocation features
> they don't need. The core functionality is now available through the
> new pci_p2pdma_enable() function that returns a p2pdma_provider
> structure.
> 
> Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> ---
>  drivers/pci/p2pdma.c       | 118 ++++++++++++++++++++++++++-----------
>  include/linux/pci-p2pdma.h |   5 ++
>  2 files changed, 89 insertions(+), 34 deletions(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 176a99232fdca..24a6c8ff88520 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -25,11 +25,12 @@ struct pci_p2pdma {
>  	struct gen_pool *pool;
>  	bool p2pmem_published;
>  	struct xarray map_types;
> +	struct p2pdma_provider mem;
>  };
>  
>  struct pci_p2pdma_pagemap {
>  	struct dev_pagemap pgmap;
> -	struct p2pdma_provider mem;
> +	struct p2pdma_provider *mem;
>  };
>  
>  static struct pci_p2pdma_pagemap *to_p2p_pgmap(struct dev_pagemap *pgmap)
> @@ -204,7 +205,7 @@ static void p2pdma_page_free(struct page *page)
>  	struct pci_p2pdma_pagemap *pgmap = to_p2p_pgmap(page_pgmap(page));
>  	/* safe to dereference while a reference is held to the percpu ref */
>  	struct pci_p2pdma *p2pdma = rcu_dereference_protected(
> -		to_pci_dev(pgmap->mem.owner)->p2pdma, 1);
> +		to_pci_dev(pgmap->mem->owner)->p2pdma, 1);
>  	struct percpu_ref *ref;
>  
>  	gen_pool_free_owner(p2pdma->pool, (uintptr_t)page_to_virt(page),
> @@ -227,44 +228,82 @@ static void pci_p2pdma_release(void *data)
>  
>  	/* Flush and disable pci_alloc_p2p_mem() */
>  	pdev->p2pdma = NULL;
> -	synchronize_rcu();
> +	if (p2pdma->pool)
> +		synchronize_rcu();
> +	xa_destroy(&p2pdma->map_types);
> +
> +	if (!p2pdma->pool)
> +		return;
>  
>  	gen_pool_destroy(p2pdma->pool);
>  	sysfs_remove_group(&pdev->dev.kobj, &p2pmem_group);
> -	xa_destroy(&p2pdma->map_types);
>  }
>  
> -static int pci_p2pdma_setup(struct pci_dev *pdev)
> +/**
> + * pci_p2pdma_enable - Enable peer-to-peer DMA support for a PCI device
> + * @pdev: The PCI device to enable P2PDMA for
> + *
> + * This function initializes the peer-to-peer DMA infrastructure for a PCI
> + * device. It allocates and sets up the necessary data structures to support
> + * P2PDMA operations, including mapping type tracking.
> + */
> +struct p2pdma_provider *pci_p2pdma_enable(struct pci_dev *pdev)
>  {
> -	int error = -ENOMEM;
>  	struct pci_p2pdma *p2p;
> +	int ret;
> +
> +	p2p = rcu_dereference_protected(pdev->p2pdma, 1);
> +	if (p2p)
> +		/* PCI device was "rebound" to the driver */
> +		return &p2p->mem;
>  
>  	p2p = devm_kzalloc(&pdev->dev, sizeof(*p2p), GFP_KERNEL);
>  	if (!p2p)
> -		return -ENOMEM;
> +		return ERR_PTR(-ENOMEM);
>  
>  	xa_init(&p2p->map_types);
> +	p2p->mem.owner = &pdev->dev;
> +	/* On all p2p platforms bus_offset is the same for all BARs */
> +	p2p->mem.bus_offset =
> +		pci_bus_address(pdev, 0) - pci_resource_start(pdev, 0);

But not all devices implement BAR0, nor is BAR0 necessarily in the
memory space, wouldn't this calculation be wrong if BAR0 were
unimplemented or an IO BAR?  Even within memory BARs I can imagine
different translations for 32 vs 64 bit, prefetch vs non-prefetch, but
per the comment I guess we're excluding those.  Thanks,

Alex