linux-kernel - Re: [PATCH 1/1] iommu/of: Fix request and enable ACS for of_iommu

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Date:   Fri, 7 May 2021 16:14:54 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Wang Xingang <wangxingang5@...wei.com>
Cc:     will@...nel.org, joro@...tes.org, bhelgaas@...gle.com,
        gregkh@...uxfoundation.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        xieyingtai@...wei.com
Subject: Re: [PATCH 1/1] iommu/of: Fix request and enable ACS for
 of_iommu_configure

On Fri, May 07, 2021 at 12:49:53PM +0000, Wang Xingang wrote:
> From: Xingang Wang <wangxingang5@...wei.com>
> 
> When request ACS for PCI device in of_iommu_configure, the pci device
> has already been scanned and added with 'pci_acs_enable=0'. So the
> pci_request_acs() in current procedure does not work for enabling ACS.
> Besides, the ACS should be enabled only if there's an IOMMU in system.
> So this fix the call of pci_request_acs() and call pci_enable_acs() to
> make sure ACS is enabled for the pci_device.

For consistency:

  s/of_iommu_configure/of_iommu_configure()/
  s/pci device/PCI device/
  s/pci_device/PCI device/

But I'm confused about what problem this fixes.  On x86, I think we
*do* set pci_acs_enable=1 in this path:

  start_kernel
    mm_init
      mem_init
        pci_iommu_alloc
          p->detect()
            detect_intel_iommu       # IOMMU_INIT_POST(detect_intel_iommu)
              pci_request_acs
                pci_acs_enable = 1

before enumerating any PCI devices.

But you mentioned pci_host_common_probe(), which I think is mostly
used on non-x86 architectures, and I'm guessing those arches detect
the IOMMU differently.

So my question is, can we figure out how to detect IOMMUs the same way
across all arches?

> Fixes: 6bf6c24720d33 ("iommu/of: Request ACS from the PCI core when
> configuring IOMMU linkage")
> Signed-off-by: Xingang Wang <wangxingang5@...wei.com>
> ---
>  drivers/iommu/of_iommu.c | 10 +++++++++-
>  drivers/pci/pci.c        |  2 +-
>  include/linux/pci.h      |  1 +
>  3 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
> index a9d2df001149..dc621861ae72 100644
> --- a/drivers/iommu/of_iommu.c
> +++ b/drivers/iommu/of_iommu.c
> @@ -205,7 +205,6 @@ const struct iommu_ops *of_iommu_configure(struct device *dev,
>  			.np = master_np,
>  		};
>  
> -		pci_request_acs();
>  		err = pci_for_each_dma_alias(to_pci_dev(dev),
>  					     of_pci_iommu_init, &info);
>  	} else {
> @@ -222,6 +221,15 @@ const struct iommu_ops *of_iommu_configure(struct device *dev,
>  		/* The fwspec pointer changed, read it again */
>  		fwspec = dev_iommu_fwspec_get(dev);
>  		ops    = fwspec->ops;
> +
> +		/*
> +		 * If we found an IOMMU and the device is pci,
> +		 * make sure we enable ACS.

s/pci/PCI/ for consistency.

> +		 */
> +		if (dev_is_pci(dev)) {
> +			pci_request_acs();
> +			pci_enable_acs(to_pci_dev(dev));
> +		}
>  	}
>  	/*
>  	 * If we have reason to believe the IOMMU driver missed the initial
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b717680377a9..4e4f98ee2870 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -926,7 +926,7 @@ static void pci_std_enable_acs(struct pci_dev *dev)
>   * pci_enable_acs - enable ACS if hardware support it
>   * @dev: the PCI device
>   */
> -static void pci_enable_acs(struct pci_dev *dev)
> +void pci_enable_acs(struct pci_dev *dev)
>  {
>  	if (!pci_acs_enable)
>  		goto disable_acs_redir;
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index c20211e59a57..e6a8bfbc9c98 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -2223,6 +2223,7 @@ static inline struct pci_dev *pcie_find_root_port(struct pci_dev *dev)
>  }
>  
>  void pci_request_acs(void);
> +void pci_enable_acs(struct pci_dev *dev);
>  bool pci_acs_enabled(struct pci_dev *pdev, u16 acs_flags);
>  bool pci_acs_path_enabled(struct pci_dev *start,
>  			  struct pci_dev *end, u16 acs_flags);
> -- 
> 2.19.1
>