RE: [PATCH v2] iommu/hyper-v: Allow hyperv irq remapping without x2apic

[Michael Kelley (LINUX)]

From: Nuno Das Neves <nunodasneves@xxxxxxxxxxxxxxxxxxx> Sent: Thursday, November 17, 2022 12:12 PM
> 
> 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@xxxxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/mshyperv.c |  6 ++++++
>  drivers/iommu/Kconfig          |  6 +++---
>  drivers/iommu/hyperv-iommu.c   | 11 ++++++++---
>  3 files changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 831613959a92..46668e255421 100644
> --- 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 dc5f7a156ff5..cf7433652db0 100644
> --- 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 e190bb8c225c..8302db7f783e 100644
> --- 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 = {
> --
> 2.25.1

Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>