Re: [PATCH v2 1/6] x86/irq: Add enumeration of NMI source reporting CPU feature

[H. Peter Anvin]

On June 11, 2024 7:32:54 PM PDT, Xin Li <xin@xxxxxxxxx> wrote:
>On 6/11/2024 9:54 AM, Jacob Pan wrote:
>> The lack of a mechanism to pinpoint the origins of Non-Maskable Interrupts
>> (NMIs) necessitates that the NMI vector 2 handler consults each NMI source
>> handler individually. This approach leads to inefficiencies, delays, and
>> the occurrence of unnecessary NMIs, thereby also constraining the potential
>> applications of NMIs.
>> 
>> A new CPU feature, known as NMI source reporting, has been introduced as
>> part of the Flexible Return and Event Delivery (FRED) spec. This feature
>> enables the NMI vector 2 handler to directly obtain information about the
>> NMI source from the FRED event data.
>> 
>> The functionality of NMI source reporting is tied to the FRED. Although it
>> is enumerated by a unique CPUID feature bit, it cannot be turned off
>> independently once FRED is activated.
>> 
>> Signed-off-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
>> ---
>> v2: Removed NMI source from static CPU ID dependency table (HPA)
>> ---
>>   arch/x86/Kconfig                         | 9 +++++++++
>>   arch/x86/include/asm/cpufeatures.h       | 1 +
>>   arch/x86/include/asm/disabled-features.h | 8 +++++++-
>>   arch/x86/kernel/traps.c                  | 4 +++-
>>   4 files changed, 20 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index 1d7122a1883e..b8b15f20b94e 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -511,12 +511,21 @@ config X86_CPU_RESCTRL
>>   config X86_FRED
>>   	bool "Flexible Return and Event Delivery"
>>   	depends on X86_64
>> +	select X86_NMI_SOURCE
>>   	help
>>   	  When enabled, try to use Flexible Return and Event Delivery
>>   	  instead of the legacy SYSCALL/SYSENTER/IDT architecture for
>>   	  ring transitions and exception/interrupt handling if the
>>   	  system supports it.
>>   +config X86_NMI_SOURCE
>
>Lets reuse X86_FRED instead of adding another hard config option. See
>below.
>
>> +	def_bool n
>> +	help
>> +	  Once enabled, information on NMI originator/source can be provided
>> +	  via FRED event data. This makes NMI processing more efficient in that
>> +	  NMI handler does not need to check for every possible source at
>> +	  runtime when NMI is delivered.
>> +
>>   config X86_BIGSMP
>>   	bool "Support for big SMP systems with more than 8 CPUs"
>>   	depends on SMP && X86_32
>
>...
>
>> diff --git a/arch/x86/include/asm/disabled-features.h b/arch/x86/include/asm/disabled-features.h
>> index c492bdc97b05..3856c4737d65 100644
>> --- a/arch/x86/include/asm/disabled-features.h
>> +++ b/arch/x86/include/asm/disabled-features.h
>> @@ -123,6 +123,12 @@
>>   # define DISABLE_FRED	(1 << (X86_FEATURE_FRED & 31))
>>   #endif
>>   +#ifdef CONFIG_X86_NMI_SOURCE
>> +# define DISABLE_NMI_SOURCE	0
>> +#else
>> +# define DISABLE_NMI_SOURCE	(1 << (X86_FEATURE_NMI_SOURCE & 31))
>> +#endif
>> +
>>   #ifdef CONFIG_KVM_AMD_SEV
>>   #define DISABLE_SEV_SNP		0
>>   #else
>> @@ -145,7 +151,7 @@
>>   #define DISABLED_MASK10	0
>>   #define DISABLED_MASK11	(DISABLE_RETPOLINE|DISABLE_RETHUNK|DISABLE_UNRET| \
>>   			 DISABLE_CALL_DEPTH_TRACKING|DISABLE_USER_SHSTK)
>> -#define DISABLED_MASK12	(DISABLE_FRED|DISABLE_LAM)
>> +#define DISABLED_MASK12	(DISABLE_FRED|DISABLE_LAM|DISABLE_NMI_SOURCE)
>>   #define DISABLED_MASK13	0
>>   #define DISABLED_MASK14	0
>>   #define DISABLED_MASK15	0
>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>> index 4fa0b17e5043..465f04e4a79f 100644
>> --- a/arch/x86/kernel/traps.c
>> +++ b/arch/x86/kernel/traps.c
>> @@ -1427,8 +1427,10 @@ early_param("fred", fred_setup);
>>     void __init trap_init(void)
>>   {
>> -	if (cpu_feature_enabled(X86_FEATURE_FRED) && !enable_fred)
>> +	if (cpu_feature_enabled(X86_FEATURE_FRED) && !enable_fred) {
>>   		setup_clear_cpu_cap(X86_FEATURE_FRED);
>> +		setup_clear_cpu_cap(X86_FEATURE_NMI_SOURCE);
>> +	}
>
>With this, no need to add DISABLE_NMI_SOURCE to disabled-features.h:
>
>1) If FRED is not available, NMI source won't be too.
>2) If FRED is available but not enabled, all features relying on FRED
>should be cleared. We probably should move the feature bits clearing
>code into a static function when more such features are added in future.
>
>>     	/* Init cpu_entry_area before IST entries are set up */
>>   	setup_cpu_entry_areas();
>
>Thanks!
>    Xin

And even if we did, FRED should not *select* NMI_SOURCE; the dependency goes the other way.