On Fri, Jan 02, 2026 at 02:02:08PM -0800, Mukesh Rathor wrote:
> 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 <[email protected]>
> ---
>
> v1: Add ifndef CONFIG_X86_FRED (thanks 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
I briefly looked up FRED and checked the code. I understand that once it
is enabled, Linux kernel doesn't setup the IDT anymore (code in
arch/x86/kernel/traps.c).
My question is, do we need to do anything when FRED is enabled?
Wei
> +/*
> + * 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;
>
> --
> 2.51.2.vfs.0.1
>
