iommu/hyper-v: Allow hyperv irq remapping without x2apic

If x2apic is not available, hyperv-iommu skips remapping
irqs. This breaks root partition which always needs irqs
remapped.

Fix this by allowing irq remapping regardless of x2apic,
and change hyperv_enable_irq_remapping() to return
IRQ_REMAP_XAPIC_MODE in case x2apic is missing.

Tested with root and non-root hyperv partitions.
Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
Reviewed-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1668715899-8971-1-git-send-email-nunodasneves@linux.microsoft.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

arch/x86/kernel/cpu/mshyperv.c

@@ -475,6 +475,12 @@ static bool __init ms_hyperv_x2apic_available(void)
  * (logically) generates MSIs directly to the system APIC irq domain.
  * There is no HPET, and PCI MSI/MSI-X interrupts are remapped by the
  * pci-hyperv host bridge.
+ *
+ * Note: for a Hyper-V root partition, this will always return false.
+ * The hypervisor doesn't expose these HYPERV_CPUID_VIRT_STACK_* cpuids by
+ * default, they are implemented as intercepts by the Windows Hyper-V stack.
+ * Even a nested root partition (L2 root) will not get them because the
+ * nested (L1) hypervisor filters them out.
  */
 static bool __init ms_hyperv_msi_ext_dest_id(void)
 {

drivers/iommu/Kconfig

@@ -474,13 +474,13 @@ config QCOM_IOMMU
 	  Support for IOMMU on certain Qualcomm SoCs.
 
 config HYPERV_IOMMU
-	bool "Hyper-V x2APIC IRQ Handling"
+	bool "Hyper-V IRQ Handling"
 	depends on HYPERV && X86
 	select IOMMU_API
 	default HYPERV
 	help
-	  Stub IOMMU driver to handle IRQs as to allow Hyper-V Linux
-	  guests to run with x2APIC mode enabled.
+	  Stub IOMMU driver to handle IRQs to support Hyper-V Linux
+	  guest and root partitions.
 
 config VIRTIO_IOMMU
 	tristate "Virtio IOMMU driver"

drivers/iommu/hyperv-iommu.c

@@ -122,9 +122,12 @@ static int __init hyperv_prepare_irq_remapping(void)
 	const char *name;
 	const struct irq_domain_ops *ops;
 
+	/*
+	 * For a Hyper-V root partition, ms_hyperv_msi_ext_dest_id()
+	 * will always return false.
+	 */
 	if (!hypervisor_is_type(X86_HYPER_MS_HYPERV) ||
-	    x86_init.hyper.msi_ext_dest_id() ||
-	    !x2apic_supported())
+	    x86_init.hyper.msi_ext_dest_id())
 		return -ENODEV;
 
 	if (hv_root_partition) {
@@ -170,7 +173,9 @@ static int __init hyperv_prepare_irq_remapping(void)
 
 static int __init hyperv_enable_irq_remapping(void)
 {
-	return IRQ_REMAP_X2APIC_MODE;
+	if (x2apic_supported())
+		return IRQ_REMAP_X2APIC_MODE;
+	return IRQ_REMAP_XAPIC_MODE;
 }
 
 struct irq_remap_ops hyperv_irq_remap_ops = {