Re: [PATCH 1/2] x86/svm: Add infrastructure for Bus Lock Threshold

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

On Tue Jan 20, 2026 at 2:12 PM CET, Andrew Cooper wrote:
> On 20/01/2026 9:53 am, Alejandro Vallejo wrote:
>> diff --git a/xen/arch/x86/hvm/svm/vmcb.h b/xen/arch/x86/hvm/svm/vmcb.h
>> index ba554a9644..85e194f247 100644
>> --- a/xen/arch/x86/hvm/svm/vmcb.h
>> +++ b/xen/arch/x86/hvm/svm/vmcb.h
>> @@ -65,6 +65,11 @@ enum GenericIntercept2bits
>>      GENERAL2_INTERCEPT_RDPRU   = 1 << 14,
>>  };
>>  
>> +/* general 2 intercepts */
>> +enum GenericIntercept3bits
>> +{
>> +    GENERAL3_INTERCEPT_BUS_LOCK_THRESH = 1 << 5,
>> +};
>
> Abbreviating thresh like this not great.
>
> For the intercept, it can probably just be called BUS_LOCK.  There's no
> other form of such intercept.
>
>>  
>>  /* control register intercepts */
>>  enum CRInterceptBits
>> @@ -289,6 +294,7 @@ enum VMEXIT_EXITCODE
>>      VMEXIT_MWAIT_CONDITIONAL= 140, /* 0x8c */
>>      VMEXIT_XSETBV           = 141, /* 0x8d */
>>      VMEXIT_RDPRU            = 142, /* 0x8e */
>> +    VMEXIT_BUSLOCK          = 165, /* 0xa5 */
>
> VMEXIT_BUS_LOCK for consistency.
>
>>      /* Remember to also update VMEXIT_NPF_PERFC! */
>>      VMEXIT_NPF              = 1024, /* 0x400, nested paging fault */
>>      /* Remember to also update SVM_PERF_EXIT_REASON_SIZE! */
>> @@ -405,7 +411,8 @@ struct vmcb_struct {
>>      u32 _exception_intercepts;  /* offset 0x08 - cleanbit 0 */
>>      u32 _general1_intercepts;   /* offset 0x0C - cleanbit 0 */
>>      u32 _general2_intercepts;   /* offset 0x10 - cleanbit 0 */
>> -    u32 res01[10];
>> +    u32 _general3_intercepts;   /* offset 0x14 - cleanbit 0 */
>> +    u32 res01[9];
>>      u16 _pause_filter_thresh;   /* offset 0x3C - cleanbit 0 */
>>      u16 _pause_filter_count;    /* offset 0x3E - cleanbit 0 */
>>      u64 _iopm_base_pa;          /* offset 0x40 - cleanbit 1 */
>> @@ -489,7 +496,10 @@ struct vmcb_struct {
>>      u64 nextrip;                /* offset 0xC8 */
>>      u8  guest_ins_len;          /* offset 0xD0 */
>>      u8  guest_ins[15];          /* offset 0xD1 */
>> -    u64 res10a[100];            /* offset 0xE0 pad to save area */
>> +    u64 res10a[8];              /* offset 0xE0 */
>> +    u16 bus_lock_thresh;        /* offset 0x120 */
>
> bus_lock_count, which is basically it's APM name anyway.
>
>> diff --git a/xen/arch/x86/include/asm/hvm/svm.h 
>> b/xen/arch/x86/include/asm/hvm/svm.h
>> index a6d7e4aed3..14fe4abf96 100644
>> --- a/xen/arch/x86/include/asm/hvm/svm.h
>> +++ b/xen/arch/x86/include/asm/hvm/svm.h
>> @@ -37,6 +37,7 @@ extern u32 svm_feature_flags;
>>  #define SVM_FEATURE_VGIF          16 /* Virtual GIF */
>>  #define SVM_FEATURE_SSS           19 /* NPT Supervisor Shadow Stacks */
>>  #define SVM_FEATURE_SPEC_CTRL     20 /* MSR_SPEC_CTRL virtualisation */
>> +#define SVM_FEATURE_BUS_LOCK_THRESH 29 /* Bus Lock Threshold */
>>  
>>  static inline bool cpu_has_svm_feature(unsigned int feat)
>>  {
>> @@ -56,6 +57,7 @@ static inline bool cpu_has_svm_feature(unsigned int feat)
>>  #define cpu_has_svm_vloadsave cpu_has_svm_feature(SVM_FEATURE_VLOADSAVE)
>>  #define cpu_has_svm_sss       cpu_has_svm_feature(SVM_FEATURE_SSS)
>>  #define cpu_has_svm_spec_ctrl cpu_has_svm_feature(SVM_FEATURE_SPEC_CTRL)
>> +#define cpu_has_bus_lock_thresh 
>> cpu_has_svm_feature(SVM_FEATURE_BUS_LOCK_THRESH)
>
> We actually discussed this on the x86 call just yesterday.  This wants
> an svm infix to match the others, and the thresh suffix can be dropped.
>
> I can fix all of these on commit.

Fine by me. Is that an implicit R-by?

Cheers,
Alejandro