PCI/MSI: Reject multi-MSI early

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@linutronix.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20221111122015.574339988@linutronix.de

drivers/pci/msi/irqdomain.c

@@ -187,6 +187,28 @@ struct irq_domain *pci_msi_create_irq_domain(struct fwnode_handle *fwnode,
 }
 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
+ */
+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

drivers/pci/msi/msi.c

@@ -347,6 +347,10 @@ static int msi_capability_init(struct pci_dev *dev, int nvec,
 	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.

drivers/pci/msi/msi.h

@@ -97,6 +97,15 @@ int __pci_enable_msix_range(struct pci_dev *dev, struct msix_entry *entries, int
 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