Re: [XEN][PATCH v3] xen: make VMTRACE support optional

[Grygorii Strashko <grygorii_strashko@epam.com>]

Hi Jan,

On 17.11.25 18:52, Jan Beulich wrote:
> On 14.11.2025 15:22, Grygorii Strashko wrote:
>> --- a/xen/arch/x86/hvm/Kconfig
>> +++ b/xen/arch/x86/hvm/Kconfig
>> @@ -35,6 +35,18 @@ config INTEL_VMX
>>   	  If your system includes a processor with Intel VT-x support, say Y.
>>   	  If in doubt, say Y.
>>   
>> +config VMTRACE
>> +    bool "HW VM tracing support"
>> +    depends on INTEL_VMX
>> +    default y
>> +    help
>> +      Enables HW VM tracing support which allows to configure HW processor
>> +      features (vmtrace_op) to enable capturing information about software
>> +      execution using dedicated hardware facilities with minimal interference
>> +      to the software being traced. The trace data can be retrieved using buffer
>> +      shared between Xen and domain
>> +      (see XENMEM_acquire_resource(XENMEM_resource_vmtrace_buf)).
> 
> Please check adjacent options above or ...
> 
>>   config HVM_FEP
>>   	bool "HVM Forced Emulation Prefix support (UNSUPPORTED)" if UNSUPPORTED
>>   	default DEBUG
> 
> ... below for how proper indentation would look like here.

There is a mix in Kconfigs - some places <Tabs> some places <Spaces> :(
Will change to <Tabs>

> 
>> --- a/xen/arch/x86/hvm/vmx/vmcs.c
>> +++ b/xen/arch/x86/hvm/vmx/vmcs.c
>> @@ -307,6 +307,7 @@ static int vmx_init_vmcs_config(bool bsp)
>>       rdmsrl(MSR_IA32_VMX_MISC, _vmx_misc_cap);
>>   
>>       /* Check whether IPT is supported in VMX operation. */
>> +#ifdef CONFIG_VMTRACE
> 
> This imo wants to move ahead of the comment. (Feels a little like I may have
> said so already, but it may well have been in the context of a different
> recent patch.)
> 
>> @@ -772,13 +775,24 @@ static inline int hvm_vmtrace_get_option(
>>   
>>       return -EOPNOTSUPP;
>>   }
>> +#else
>> +/*
>> + * Function declaration(s) here are used without definition(s) to make compiler
>> + * happy when VMTRACE=n, compiler DCE will eliminate unused code.
>> + */
>> +int hvm_vmtrace_output_position(struct vcpu *v, uint64_t *pos);
>> +#endif
>>   
>>   static inline int hvm_vmtrace_reset(struct vcpu *v)
>>   {
>> +#ifdef CONFIG_VMTRACE
>>       if ( hvm_funcs.vmtrace_reset )
>>           return alternative_call(hvm_funcs.vmtrace_reset, v);
>>   
>>       return -EOPNOTSUPP;
>> +#else
>> +    return -EOPNOTSUPP;
>> +#endif
> 
> No #else please and the #endif moved up.
> 
>> --- a/xen/arch/x86/vm_event.c
>> +++ b/xen/arch/x86/vm_event.c
>> @@ -253,7 +253,8 @@ void vm_event_fill_regs(vm_event_request_t *req)
>>       req->data.regs.x86.shadow_gs = ctxt.shadow_gs;
>>       req->data.regs.x86.dr6 = ctxt.dr6;
>>   
>> -    if ( hvm_vmtrace_output_position(curr, &req->data.regs.x86.vmtrace_pos) != 1 )
>> +    if ( IS_ENABLED(CONFIG_VMTRACE) &&
>> +         hvm_vmtrace_output_position(curr, &req->data.regs.x86.vmtrace_pos) != 1 )
> 
> Would be nice if the too-long-line issue here was also address, when the line
> needs touching anyway.

I left it as is for better readability as an exception.
Will below be ok:

      if ( IS_ENABLED(CONFIG_VMTRACE) &&
-         hvm_vmtrace_output_position(curr, &req->data.regs.x86.vmtrace_pos) != 1 )
+         hvm_vmtrace_output_position(curr,
+                                     &req->data.regs.x86.vmtrace_pos) != 1 )
          req->data.regs.x86.vmtrace_pos = ~0;

-- 
Best regards,
-grygorii