linux-kernel - Re: [patch 34/39] PCI/MSI: Reject multi-MSI early

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Date:   Wed, 16 Nov 2022 10:31:18 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Ashok Raj <ashok.raj@...el.com>, Jon Mason <jdmason@...zu.us>,
        Allen Hubbe <allenbh@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        linuxppc-dev@...ts.ozlabs.org,
        "Ahmed S. Darwish" <darwi@...utronix.de>,
        Reinette Chatre <reinette.chatre@...el.com>
Subject: Re: [patch 34/39] PCI/MSI: Reject multi-MSI early

On Fri, Nov 11, 2022 at 02:55:09PM +0100, Thomas Gleixner wrote:
> When hierarchical MSI interrupt domains are enabled then there is no point
> to do tons of work and detect the missing support for multi-MSI late in the
> allocation path.
> 
> Just query the domain feature flags right away. The query function is going
> to be used for other purposes later and has a mode argument which influences
> the result:
> 
>   ALLOW_LEGACY returns true when:
>      - there is no irq domain attached (legacy support)
>      - there is a irq domain attached which has the feature flag set
> 
>   DENY_LEGACY returns only true when:
>      - there is a irq domain attached which has the feature flag set
> 
> This allows to use the function universally without ifdeffery in the
> calling code.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>

> ---
>  drivers/pci/msi/irqdomain.c |   22 ++++++++++++++++++++++
>  drivers/pci/msi/msi.c       |    4 ++++
>  drivers/pci/msi/msi.h       |    9 +++++++++
>  3 files changed, 35 insertions(+)
> 
> --- a/drivers/pci/msi/irqdomain.c
> +++ b/drivers/pci/msi/irqdomain.c
> @@ -187,6 +187,28 @@ struct irq_domain *pci_msi_create_irq_do
>  }
>  EXPORT_SYMBOL_GPL(pci_msi_create_irq_domain);
>  
> +/**
> + * pci_msi_domain_supports - Check for support of a particular feature flag
> + * @pdev:		The PCI device to operate on
> + * @feature_mask:	The feature mask to check for (full match)
> + * @mode:		If ALLOW_LEGACY this grants the feature when there is no irq domain
> + *			associated to the device. If DENY_LEGACY the lack of an irq domain
> + *			makes the feature unsupported

Looks like some of these might be wider than 80 columns, which I think
was the typical width of this file.

> + */
> +bool pci_msi_domain_supports(struct pci_dev *pdev, unsigned int feature_mask,
> +			     enum support_mode mode)
> +{
> +	struct msi_domain_info *info;
> +	struct irq_domain *domain;
> +
> +	domain = dev_get_msi_domain(&pdev->dev);
> +
> +	if (!domain || !irq_domain_is_hierarchy(domain))
> +		return mode == ALLOW_LEGACY;
> +	info = domain->host_data;
> +	return (info->flags & feature_mask) == feature_mask;
> +}
> +
>  /*
>   * Users of the generic MSI infrastructure expect a device to have a single ID,
>   * so with DMA aliases we have to pick the least-worst compromise. Devices with
> --- a/drivers/pci/msi/msi.c
> +++ b/drivers/pci/msi/msi.c
> @@ -347,6 +347,10 @@ static int msi_capability_init(struct pc
>  	struct msi_desc *entry;
>  	int ret;
>  
> +	/* Reject multi-MSI early on irq domain enabled architectures */
> +	if (nvec > 1 && !pci_msi_domain_supports(dev, MSI_FLAG_MULTI_PCI_MSI, ALLOW_LEGACY))
> +		return 1;
> +
>  	/*
>  	 * Disable MSI during setup in the hardware, but mark it enabled
>  	 * so that setup code can evaluate it.
> --- a/drivers/pci/msi/msi.h
> +++ b/drivers/pci/msi/msi.h
> @@ -97,6 +97,15 @@ int __pci_enable_msix_range(struct pci_d
>  void __pci_restore_msi_state(struct pci_dev *dev);
>  void __pci_restore_msix_state(struct pci_dev *dev);
>  
> +/* irq_domain related functionality */
> +
> +enum support_mode {
> +	ALLOW_LEGACY,
> +	DENY_LEGACY,
> +};
> +
> +bool pci_msi_domain_supports(struct pci_dev *dev, unsigned int feature_mask, enum support_mode mode);
> +
>  /* Legacy (!IRQDOMAIN) fallbacks */
>  
>  #ifdef CONFIG_PCI_MSI_ARCH_FALLBACKS
>