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

[Mukesh R <mrathor@linux.microsoft.com>]

On 2/20/26 10:45, Wei Liu wrote:
> On Fri, Feb 20, 2026 at 05:14:26PM +0000, Michael Kelley wrote:
>> From: Mukesh R <mrathor@linux.microsoft.com> Sent: Tuesday, February 17, 2026 3:12 PM
>>>
>>> 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@linux.microsoft.com>
>>> ---
>>>
>>> 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);
>>
>> I'm a little late to the party here, but I've always seen Intel interrupt vectors
>> displayed as 2-digit hex numbers. This info message is displaying decimal,
>> which is atypical and will probably be confusing.
> 
> Noted. The pull request to Linus has been sent. We will change the
> format in a follow up patch.

Well, there is no 0x prefix, so should not be confusing, but no big
deal, whatever.....

Thanks,
-Mukesh