linux-kernel - Re: [PATCH v1] x86/hyperv: Reserve 3 interrupt vectors used exclusively by mshv

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Message-ID: <72687e18-ecdc-438a-9ea6-fd935f599b27@zytor.com>
Date: Tue, 10 Feb 2026 12:33:17 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Mukesh Rathor <mrathor@...ux.microsoft.com>, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, longli@...rosoft.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org
Subject: Re: [PATCH v1] x86/hyperv: Reserve 3 interrupt vectors used
 exclusively by mshv

On 2026-01-02 14:02, Mukesh Rathor wrote:
> 
> v1: Add ifndef CONFIG_X86_FRED (thanks hpa)
> 

It just clicked in my brain.

This must be cpu_feature_enabled() not a static #ifndef. Just because the
kernel is compiled with FRED support doesn't mean that it is *using* FRED!

	-hpa


>  arch/x86/kernel/cpu/mshyperv.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 579fb2c64cfd..8ef4ca6733ac 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -478,6 +478,27 @@ int hv_get_hypervisor_version(union hv_hypervisor_version_info *info)
>  }
>  EXPORT_SYMBOL_GPL(hv_get_hypervisor_version);
>  
> +#ifndef CONFIG_X86_FRED
> +/*
> + * Reserve vectors hard coded in the hypervisor. If used outside, the hypervisor
> + * will crash or hang or 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 (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);
> +}
> +#endif          /* CONFIG_X86_FRED */
> +
>  static void __init ms_hyperv_init_platform(void)
>  {
>  	int hv_max_functions_eax, eax;
> @@ -510,6 +531,11 @@ 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 */
> +
>  	if (cc_platform_has(CC_ATTR_SNP_SECURE_AVIC))
>  		ms_hyperv.hints |= HV_DEPRECATING_AEOI_RECOMMENDED;
>