Re: [PATCH] x86/hyperv: reserve more vectors

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

On 8/10/26 08:35, Michael Kelley wrote:
> From: wei.liu@kernel.org <wei.liu@kernel.org> Sent: Wednesday, July 29, 2026 5:50 PM
>>
>> Microsoft Hypervisor delivers three vectors to the NT HAL running in the
>> root partition and refuses to map a device interrupt to any of them when
>> interrupt remapping is not available in the system. As of writing, the
> 
> s/of writing/of this writing/
> 
>> nested MSHV setup has no interrupt remapping capability.
>>
>> The three vectors are:
>>
>>    HAL_NT_APC_VECTOR		0x1F
>>    HAL_NT_DPC_VECTOR		0x2F
>>    HAL_NT_CLOCK_IPI_VECTOR	0xD2
>>
>> 0x1F is below FIRST_EXTERNAL_VECTOR so the vector allocator never hands
>> it out, but 0x2F and 0xD2 are both inside the allocatable range and are
>> handed out once enough vectors are in use. Mapping such an interrupt
>> then fails with HV_STATUS_INVALID_PARAMETER, and the interrupt is never
>> delivered.
>>
>> Reserve all three next to the hypervisor debug vectors that are already
>> kept out of the allocator's hands.
>>
>> Signed-off-by: Wei Liu <wei.liu@kernel.org>
>> ---
>>   arch/x86/kernel/cpu/mshyperv.c | 21 ++++++++++++++++++---
>>   1 file changed, 18 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
>> index 185d4f677ec0..229c7377a980 100644
>> --- a/arch/x86/kernel/cpu/mshyperv.c
>> +++ b/arch/x86/kernel/cpu/mshyperv.c
>> @@ -502,17 +502,32 @@ static void hv_reserve_irq_vectors(void)
>>   	#define HYPERV_DBG_ASSERT_VECTOR	0x2C
>>   	#define HYPERV_DBG_SERVICE_VECTOR	0x2D
>>
>> +	/*
>> +	 * The hypervisor delivers these three to the NT HAL and refuses to
>> +	 * map a device interrupt to any of them.
>> +	 *
>> +	 * The hypervisor will provide a hint in the future when these
>> +	 * vectors become available to use.
>> +	 */
>> +	#define HAL_NT_APC_VECTOR		0x1F
>> +	#define HAL_NT_DPC_VECTOR		0x2F
>> +	#define HAL_NT_CLOCK_IPI_VECTOR		0xD2
>> +
>>   	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))
>> +	    test_and_set_bit(HYPERV_DBG_FASTFAIL_VECTOR, system_vectors) ||
>> +	    test_and_set_bit(HAL_NT_APC_VECTOR, system_vectors) ||
>> +	    test_and_set_bit(HAL_NT_DPC_VECTOR, system_vectors) ||
>> +	    test_and_set_bit(HAL_NT_CLOCK_IPI_VECTOR, system_vectors))
> 
> Calling test_and_set_bit() explicitly for each of the 6 vectors seems right
> on the border of where it would be better to declare a static array with
> the vectors and loop through the array. The loop could also populate the
> string to be output in the pr_info() statement below. But leaving it explicit
> as you have done is probably simpler unless the vector list grows again.

Simpler and better imo, but moreover, we want to discourage the
list enlarging, if anything, i hope going forward we can shrink it
back to absolutely minimum.

Thanks,
-Mukesh


> Reviewed-by: Michael Kelley <mhklinux@outlook.com>
> 
>>   		BUG();
>>
>> -	pr_info("Hyper-V: reserve vectors: 0x%x 0x%x 0x%x\n",
>> +	pr_info("Hyper-V: reserve vectors: 0x%x 0x%x 0x%x 0x%x 0x%x 0x%x\n",
>>   		HYPERV_DBG_ASSERT_VECTOR, HYPERV_DBG_SERVICE_VECTOR,
>> -		HYPERV_DBG_FASTFAIL_VECTOR);
>> +		HYPERV_DBG_FASTFAIL_VECTOR, HAL_NT_APC_VECTOR,
>> +		HAL_NT_DPC_VECTOR, HAL_NT_CLOCK_IPI_VECTOR);
>>   }
>>
>>   static void __init ms_hyperv_init_platform(void)
>> --
>> 2.53.0
>>
>