Re: [PATCH v2] x86/hyperv: Reserve 3 interrupt vectors used exclusively by mshv

[Wei Liu]

On Tue, Feb 17, 2026 at 03:11:58PM -0800, Mukesh R wrote:
> From: Mukesh Rathor <mrathor@xxxxxxxxxxxxxxxxxxx>
> 
> MSVC compiler, used to compile the Microsoft Hyper-V hypervisor currently,
> has an assert intrinsic that uses interrupt vector 0x29 to create an
> exception. This will cause hypervisor to then crash and collect core. As
> such, if this interrupt number is assigned to a device by Linux and the
> device generates it, hypervisor will crash. There are two other such
> vectors hard coded in the hypervisor, 0x2C and 0x2D for debug purposes.
> Fortunately, the three vectors are part of the kernel driver space and
> that makes it feasible to reserve them early so they are not assigned
> later.
> 
> Signed-off-by: Mukesh Rathor <mrathor@xxxxxxxxxxxxxxxxxxx>
> ---
> 
> v1: Add ifndef CONFIG_X86_FRED (thanks hpa)
> v2: replace ifndef with cpu_feature_enabled() (thanks hpa and tglx)
> 
>  arch/x86/kernel/cpu/mshyperv.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 579fb2c64cfd..88ca127dc6d4 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -478,6 +478,28 @@ int hv_get_hypervisor_version(union hv_hypervisor_version_info *info)
>  }
>  EXPORT_SYMBOL_GPL(hv_get_hypervisor_version);
>  
> +/*
> + * Reserve vectors hard coded in the hypervisor. If used outside, the hypervisor
> + * will either crash or hang or attempt to break into debugger.
> + */
> +static void hv_reserve_irq_vectors(void)
> +{
> +	#define HYPERV_DBG_FASTFAIL_VECTOR	0x29
> +	#define HYPERV_DBG_ASSERT_VECTOR	0x2C
> +	#define HYPERV_DBG_SERVICE_VECTOR	0x2D
> +
> +	if (cpu_feature_enabled(X86_FEATURE_FRED))
> +		return;
> +
> +	if (test_and_set_bit(HYPERV_DBG_ASSERT_VECTOR, system_vectors) ||
> +	    test_and_set_bit(HYPERV_DBG_SERVICE_VECTOR, system_vectors) ||
> +	    test_and_set_bit(HYPERV_DBG_FASTFAIL_VECTOR, system_vectors))
> +		BUG();
> +
> +	pr_info("Hyper-V:reserve vectors: %d %d %d\n", HYPERV_DBG_ASSERT_VECTOR,
> +		HYPERV_DBG_SERVICE_VECTOR, HYPERV_DBG_FASTFAIL_VECTOR);
> +}
> +
>  static void __init ms_hyperv_init_platform(void)
>  {
>  	int hv_max_functions_eax, eax;
> @@ -510,6 +532,11 @@ static void __init ms_hyperv_init_platform(void)
>  
>  	hv_identify_partition_type();
>  
> +#ifndef CONFIG_X86_FRED
> +	if (hv_root_partition())
> +		hv_reserve_irq_vectors();
> +#endif	/* CONFIG_X86_FRED */
> +

On a CONFIG_X86_FRED=y system, this call is skipped. However, the kernel
may not have FRED active, so the vectors should still be reserved.

I think the function should always be called.

Wei

>  	if (cc_platform_has(CC_ATTR_SNP_SECURE_AVIC))
>  		ms_hyperv.hints |= HV_DEPRECATING_AEOI_RECOMMENDED;
>  
> -- 
> 2.51.2.vfs.0.1
> 
>