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.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 8316139..46668e2 100644 (file)
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/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)
 {

diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index dc5f7a1..cf74336 100644 (file)
--- a/drivers/iommu/Kconfig
+++ b/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"

diff --git a/drivers/iommu/hyperv-iommu.c b/drivers/iommu/hyperv-iommu.c
index e190bb8..8302db7 100644 (file)
--- a/drivers/iommu/hyperv-iommu.c
+++ b/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 = {