linux-kernel - Re: [PATCH] ACPI: VIOT: Fix ACS setup

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Date:   Wed, 29 Jun 2022 10:14:01 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, jroedel@...e.de,
        linux-acpi@...r.kernel.org, iommu@...ts.linux-foundation.org,
        rafael@...nel.org, lenb@...nel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux.dev
Subject: Re: [PATCH] ACPI: VIOT: Fix ACS setup

Hi Eric,

On Mon, Jun 27, 2022 at 02:55:34PM +0200, Eric Auger wrote:
> Currently acpi_viot_init() gets called after the pci
> device has been scanned and pci_enable_acs() has been called.
> So pci_request_acs() fails to be taken into account leading
> to wrong single iommu group topologies when dealing with
> multi-function root ports for instance.
> 
> We cannot simply move the acpi_viot_init() earlier, similarly
> as the IORT init because the VIOT parsing relies on the pci
> scan. However we can detect VIOT is present earlier and in
> such a case, request ACS. Introduce a new acpi_viot_early_init()
> routine that allows to call pci_request_acs() before the scan.
> 
> Fixes: 3cf485540e7b ("ACPI: Add driver for the VIOT table")
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> Reported-by: Jin Liu <jinl@...hat.com>

Thanks for the fix, the patch makes sense and fixes the issue.

I wondered whether we should keep the logic where we only request ACS if
an IOMMU is found to manage a PCI range, but I can't see any harm in
requesting it regardless (plus there is a precedent with AMD IOMMU).
I could imagine some VMM wanting to only put an IOMMU in front of its MMIO
devices and leave PCI to roam free, but that seems like a stretch.

There is another issue with the existing code, though: we can't call
pci_request_acs() when CONFIG_PCI is disabled because no stub is defined.
Could you wrap the call in an #ifdef?

> ---
>  drivers/acpi/bus.c        |  1 +
>  drivers/acpi/viot.c       | 23 +++++++++++++++++------
>  include/linux/acpi_viot.h |  2 ++
>  3 files changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
> index 86fa61a21826..906ad8153fd9 100644
> --- a/drivers/acpi/bus.c
> +++ b/drivers/acpi/bus.c
> @@ -1400,6 +1400,7 @@ static int __init acpi_init(void)
>  
>  	pci_mmcfg_late_init();
>  	acpi_iort_init();
> +	acpi_viot_early_init();
>  	acpi_hest_init();
>  	acpi_ghes_init();
>  	acpi_scan_init();
> diff --git a/drivers/acpi/viot.c b/drivers/acpi/viot.c
> index d2256326c73a..3c1be123e4d6 100644
> --- a/drivers/acpi/viot.c
> +++ b/drivers/acpi/viot.c
> @@ -248,6 +248,23 @@ static int __init viot_parse_node(const struct acpi_viot_header *hdr)
>  	return ret;
>  }
>  
> +/**
> + * acpi_viot_early_init - Test the presence of VIOT and enable ACS
> + *
> + * If the VIOT does exist, ACS must be enabled. This cannot be
> + * done in acpi_viot_init() which is called after the bus scan
> + */
> +void __init acpi_viot_early_init(void)
> +{
> +	acpi_status status;
> +	struct acpi_table_header *hdr;
> +
> +	status = acpi_get_table(ACPI_SIG_VIOT, 0, &hdr);
> +	if (!ACPI_FAILURE(status))
> +		pci_request_acs();
> +	acpi_put_table(hdr);

I'd rather not call acpi_put_table() in case of failure. I know it is
handled but it looks fragile and I couldn't find any other user of
acpi_get_table() doing this.

> +}
> +
>  /**
>   * acpi_viot_init - Parse the VIOT table
>   *
> @@ -319,12 +336,6 @@ static int viot_pci_dev_iommu_init(struct pci_dev *pdev, u16 dev_id, void *data)
>  			epid = ((domain_nr - ep->segment_start) << 16) +
>  				dev_id - ep->bdf_start + ep->endpoint_id;
>  
> -			/*
> -			 * If we found a PCI range managed by the viommu, we're
> -			 * the one that has to request ACS.
> -			 */
> -			pci_request_acs();
> -
>  			return viot_dev_iommu_init(&pdev->dev, ep->viommu,
>  						   epid);
>  		}
> diff --git a/include/linux/acpi_viot.h b/include/linux/acpi_viot.h
> index 1eb8ee5b0e5f..e58d60f8ff2e 100644
> --- a/include/linux/acpi_viot.h
> +++ b/include/linux/acpi_viot.h
> @@ -6,10 +6,12 @@
>  #include <linux/acpi.h>
>  
>  #ifdef CONFIG_ACPI_VIOT
> +void __init acpi_viot_early_init(void);
>  void __init acpi_viot_init(void);
>  int viot_iommu_configure(struct device *dev);
>  #else
>  static inline void acpi_viot_init(void) {}
> +static inline void acpi_viot_early_init(void) {}

nit: different declaration order

Thanks,
Jean


>  static inline int viot_iommu_configure(struct device *dev)
>  {
>  	return -ENODEV;
> -- 
> 2.35.3
>