From: Grygorii Strashko <grygorii_strashko@epam.com>

The VMTRACE feature is depends on Platform/Arch HW and code support and now
can be used only on x86 HVM with Intel VT-x (INTEL_VMX) enabled.
This makes VMTRACE support optional by introducing HVM Kconfig option:
- CONFIG_VMTRACE to enable/disable the feature.

Signed-off-by: Grygorii Strashko <grygorii_strashko@epam.com>
---
changes in v3:
- drop vmtrace stubs for HVM=n case from hvm.h (VMTRACE depnds on HVM)
- hvm_vmtrace_reset() fix return err code
- add comment about using func declaration without definition

changes in v2:
 - fix comments from Jan Beulich
 - move CONFIG_VMTRACE in HVM
 - drop HAS_VMTRACE

v2:
 https://patchwork.kernel.org/project/xen-devel/patch/20251112202442.3879997-1-grygorii_strashko@epam.com/ 
v1:
 https://patchwork.kernel.org/project/xen-devel/patch/20251031212005.1338212-1-grygorii_strashko@epam.com/

 xen/arch/x86/domctl.c                   |  4 +++
 xen/arch/x86/hvm/Kconfig                | 12 +++++++++
 xen/arch/x86/hvm/vmx/vmcs.c             |  2 ++
 xen/arch/x86/hvm/vmx/vmx.c              | 11 ++++++++
 xen/arch/x86/include/asm/guest-msr.h    |  2 ++
 xen/arch/x86/include/asm/hvm/hvm.h      | 36 ++++++++++---------------
 xen/arch/x86/include/asm/hvm/vmx/vmcs.h |  2 ++
 xen/arch/x86/mm/mem_sharing.c           |  2 ++
 xen/arch/x86/vm_event.c                 |  7 ++---
 xen/common/domain.c                     | 10 +++++++
 xen/common/memory.c                     |  6 +++++
 xen/common/vm_event.c                   |  3 ++-
 xen/include/xen/domain.h                |  4 +++
 xen/include/xen/sched.h                 |  4 +++
 14 files changed, 79 insertions(+), 26 deletions(-)

diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
index 6153e3c07e2d..d9521808dcba 100644
--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -154,6 +154,7 @@ void arch_get_domain_info(const struct domain *d,
 static int do_vmtrace_op(struct domain *d, struct xen_domctl_vmtrace_op *op,
                          XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
 {
+#ifdef CONFIG_VMTRACE
     struct vcpu *v;
     int rc;
 
@@ -198,6 +199,9 @@ static int do_vmtrace_op(struct domain *d, struct xen_domctl_vmtrace_op *op,
     vcpu_unpause(v);
 
     return rc;
+#else
+    return -EOPNOTSUPP;
+#endif
 }
 
 #define MAX_IOPORTS 0x10000
diff --git a/xen/arch/x86/hvm/Kconfig b/xen/arch/x86/hvm/Kconfig
index c1a131d1851a..5c47a45c9350 100644
--- 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)).
+
 config HVM_FEP
 	bool "HVM Forced Emulation Prefix support (UNSUPPORTED)" if UNSUPPORTED
 	default DEBUG
diff --git a/xen/arch/x86/hvm/vmx/vmcs.c b/xen/arch/x86/hvm/vmx/vmcs.c
index cd5ac8a5f0e3..2fffc2006ab0 100644
--- 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
     if ( bsp )
         vmtrace_available = cpu_has_proc_trace &&
                             (_vmx_misc_cap & VMX_MISC_PROC_TRACE);
@@ -317,6 +318,7 @@ static int vmx_init_vmcs_config(bool bsp)
                smp_processor_id());
         return -EINVAL;
     }
+#endif
 
     if ( caps.cpu_based_exec_control & CPU_BASED_ACTIVATE_SECONDARY_CONTROLS )
     {
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index ee6977e94127..3f0e113b0b67 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -622,6 +622,7 @@ static void cf_check domain_creation_finished(struct domain *d)
 
 static void vmx_init_ipt(struct vcpu *v)
 {
+#ifdef CONFIG_VMTRACE
     unsigned int size = v->domain->vmtrace_size;
 
     if ( !size )
@@ -632,6 +633,7 @@ static void vmx_init_ipt(struct vcpu *v)
     ASSERT(size >= PAGE_SIZE && (size & (size - 1)) == 0);
 
     v->arch.msrs->rtit.output_limit = size - 1;
+#endif
 }
 
 static int cf_check vmx_vcpu_initialise(struct vcpu *v)
@@ -724,11 +726,13 @@ static void vmx_save_guest_msrs(struct vcpu *v)
      */
     v->arch.hvm.vmx.shadow_gs = read_gs_shadow();
 
+#ifdef CONFIG_VMTRACE
     if ( v->arch.hvm.vmx.ipt_active )
     {
         rdmsrl(MSR_RTIT_OUTPUT_MASK, msrs->rtit.output_mask);
         rdmsrl(MSR_RTIT_STATUS, msrs->rtit.status);
     }
+#endif
 
     if ( cp->feat.pks )
         msrs->pkrs = rdpkrs_and_cache();
@@ -747,12 +751,14 @@ static void vmx_restore_guest_msrs(struct vcpu *v)
     if ( cpu_has_msr_tsc_aux )
         wrmsr_tsc_aux(msrs->tsc_aux);
 
+#ifdef CONFIG_VMTRACE
     if ( v->arch.hvm.vmx.ipt_active )
     {
         wrmsrl(MSR_RTIT_OUTPUT_BASE, page_to_maddr(v->vmtrace.pg));
         wrmsrl(MSR_RTIT_OUTPUT_MASK, msrs->rtit.output_mask);
         wrmsrl(MSR_RTIT_STATUS, msrs->rtit.status);
     }
+#endif
 
     if ( cp->feat.pks )
         wrpkrs(msrs->pkrs);
@@ -2626,6 +2632,7 @@ static bool cf_check vmx_get_pending_event(
     return true;
 }
 
+#ifdef CONFIG_VMTRACE
 /*
  * We only let vmtrace agents see and modify a subset of bits in MSR_RTIT_CTL.
  * These all pertain to data-emitted into the trace buffer(s).  Must not
@@ -2768,6 +2775,7 @@ static int cf_check vmtrace_reset(struct vcpu *v)
     v->arch.msrs->rtit.status = 0;
     return 0;
 }
+#endif
 
 static uint64_t cf_check vmx_get_reg(struct vcpu *v, unsigned int reg)
 {
@@ -2940,11 +2948,14 @@ static struct hvm_function_table __initdata_cf_clobber vmx_function_table = {
     .altp2m_vcpu_emulate_ve = vmx_vcpu_emulate_ve,
     .altp2m_vcpu_emulate_vmfunc = vmx_vcpu_emulate_vmfunc,
 #endif
+
+#ifdef CONFIG_VMTRACE
     .vmtrace_control = vmtrace_control,
     .vmtrace_output_position = vmtrace_output_position,
     .vmtrace_set_option = vmtrace_set_option,
     .vmtrace_get_option = vmtrace_get_option,
     .vmtrace_reset = vmtrace_reset,
+#endif
 
     .get_reg = vmx_get_reg,
     .set_reg = vmx_set_reg,
diff --git a/xen/arch/x86/include/asm/guest-msr.h b/xen/arch/x86/include/asm/guest-msr.h
index 5f0cb0a93995..702f47fe1e16 100644
--- a/xen/arch/x86/include/asm/guest-msr.h
+++ b/xen/arch/x86/include/asm/guest-msr.h
@@ -50,6 +50,7 @@ struct vcpu_msrs
         };
     } misc_features_enables;
 
+#ifdef CONFIG_VMTRACE
     /*
      * 0x00000560 ... 57x - MSR_RTIT_*
      *
@@ -81,6 +82,7 @@ struct vcpu_msrs
             };
         };
     } rtit;
+#endif
 
     /*
      * 0x000006e1 - MSR_PKRS - Protection Key Supervisor.
diff --git a/xen/arch/x86/include/asm/hvm/hvm.h b/xen/arch/x86/include/asm/hvm/hvm.h
index 7412256a2dab..93da4dd2dc4b 100644
--- a/xen/arch/x86/include/asm/hvm/hvm.h
+++ b/xen/arch/x86/include/asm/hvm/hvm.h
@@ -234,12 +234,14 @@ struct hvm_function_table {
     int (*altp2m_vcpu_emulate_vmfunc)(const struct cpu_user_regs *regs);
 #endif
 
+#ifdef CONFIG_VMTRACE
     /* vmtrace */
     int (*vmtrace_control)(struct vcpu *v, bool enable, bool reset);
     int (*vmtrace_output_position)(struct vcpu *v, uint64_t *pos);
     int (*vmtrace_set_option)(struct vcpu *v, uint64_t key, uint64_t value);
     int (*vmtrace_get_option)(struct vcpu *v, uint64_t key, uint64_t *value);
     int (*vmtrace_reset)(struct vcpu *v);
+#endif
 
     uint64_t (*get_reg)(struct vcpu *v, unsigned int reg);
     void (*set_reg)(struct vcpu *v, unsigned int reg, uint64_t val);
@@ -738,6 +740,7 @@ static inline bool altp2m_vcpu_emulate_ve(struct vcpu *v)
 bool altp2m_vcpu_emulate_ve(struct vcpu *v);
 #endif /* CONFIG_ALTP2M */
 
+#ifdef CONFIG_VMTRACE
 static inline int hvm_vmtrace_control(struct vcpu *v, bool enable, bool reset)
 {
     if ( hvm_funcs.vmtrace_control )
@@ -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
 }
 
 /*
@@ -934,28 +948,6 @@ static inline bool hvm_has_set_descriptor_access_exiting(void)
     return false;
 }
 
-static inline int hvm_vmtrace_control(struct vcpu *v, bool enable, bool reset)
-{
-    return -EOPNOTSUPP;
-}
-
-static inline int hvm_vmtrace_output_position(struct vcpu *v, uint64_t *pos)
-{
-    return -EOPNOTSUPP;
-}
-
-static inline int hvm_vmtrace_set_option(
-    struct vcpu *v, uint64_t key, uint64_t value)
-{
-    return -EOPNOTSUPP;
-}
-
-static inline int hvm_vmtrace_get_option(
-    struct vcpu *v, uint64_t key, uint64_t *value)
-{
-    return -EOPNOTSUPP;
-}
-
 static inline uint64_t hvm_get_reg(struct vcpu *v, unsigned int reg)
 {
     ASSERT_UNREACHABLE();
diff --git a/xen/arch/x86/include/asm/hvm/vmx/vmcs.h b/xen/arch/x86/include/asm/hvm/vmx/vmcs.h
index 8ff7c8045fc6..879ec10cefd0 100644
--- a/xen/arch/x86/include/asm/hvm/vmx/vmcs.h
+++ b/xen/arch/x86/include/asm/hvm/vmx/vmcs.h
@@ -154,8 +154,10 @@ struct vmx_vcpu {
     /* Do we need to tolerate a spurious EPT_MISCONFIG VM exit? */
     bool                 ept_spurious_misconfig;
 
+#ifdef CONFIG_VMTRACE
     /* Processor Trace configured and enabled for the vcpu. */
     bool                 ipt_active;
+#endif
 
     /* Is the guest in real mode? */
     uint8_t              vmx_realmode;
diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index 4787b2796479..074f1b2562b3 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -1888,7 +1888,9 @@ static int fork(struct domain *cd, struct domain *d)
         domain_pause(d);
         cd->max_pages = d->max_pages;
         *cd->arch.cpu_policy = *d->arch.cpu_policy;
+#ifdef CONFIG_VMTRACE
         cd->vmtrace_size = d->vmtrace_size;
+#endif
         cd->parent = d;
     }
 
diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
index fc349270b9c5..e9ac1d497594 100644
--- 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 )
         req->data.regs.x86.vmtrace_pos = ~0;
 #endif
 }
@@ -303,12 +304,12 @@ void vm_event_emulate_check(struct vcpu *v, vm_event_response_t *rsp)
 #endif
 }
 
+#ifdef CONFIG_VMTRACE
 void vm_event_reset_vmtrace(struct vcpu *v)
 {
-#ifdef CONFIG_HVM
     hvm_vmtrace_reset(v);
-#endif
 }
+#endif
 
 /*
  * Local variables:
diff --git a/xen/common/domain.c b/xen/common/domain.c
index e13e01c1d272..ce4f55339fb4 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -136,7 +136,9 @@ struct vcpu *idle_vcpu[NR_CPUS] __read_mostly;
 
 vcpu_info_t dummy_vcpu_info;
 
+#ifdef CONFIG_VMTRACE
 bool __read_mostly vmtrace_available;
+#endif
 
 bool __read_mostly vpmu_is_available;
 
@@ -318,6 +320,7 @@ static void vcpu_info_reset(struct vcpu *v)
 
 static void vmtrace_free_buffer(struct vcpu *v)
 {
+#ifdef CONFIG_VMTRACE
     const struct domain *d = v->domain;
     struct page_info *pg = v->vmtrace.pg;
     unsigned int i;
@@ -332,10 +335,12 @@ static void vmtrace_free_buffer(struct vcpu *v)
         put_page_alloc_ref(&pg[i]);
         put_page_and_type(&pg[i]);
     }
+#endif
 }
 
 static int vmtrace_alloc_buffer(struct vcpu *v)
 {
+#ifdef CONFIG_VMTRACE
     struct domain *d = v->domain;
     struct page_info *pg;
     unsigned int i;
@@ -377,6 +382,9 @@ static int vmtrace_alloc_buffer(struct vcpu *v)
     }
 
     return -ENODATA;
+#else
+    return 0;
+#endif
 }
 
 /*
@@ -825,7 +833,9 @@ struct domain *domain_create(domid_t domid,
         ASSERT(!config->altp2m.nr);
 #endif
 
+#ifdef CONFIG_VMTRACE
         d->vmtrace_size = config->vmtrace_size;
+#endif
     }
 
     /* Sort out our idea of is_control_domain(). */
diff --git a/xen/common/memory.c b/xen/common/memory.c
index 3688e6dd5032..66dc7f7a0a41 100644
--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -1155,8 +1155,10 @@ static unsigned int resource_max_frames(const struct domain *d,
     case XENMEM_resource_ioreq_server:
         return ioreq_server_max_frames(d);
 
+#ifdef CONFIG_VMTRACE
     case XENMEM_resource_vmtrace_buf:
         return d->vmtrace_size >> PAGE_SHIFT;
+#endif
 
     default:
         return 0;
@@ -1198,6 +1200,7 @@ static int acquire_ioreq_server(struct domain *d,
 #endif
 }
 
+#ifdef CONFIG_VMTRACE
 static int acquire_vmtrace_buf(
     struct domain *d, unsigned int id, unsigned int frame,
     unsigned int nr_frames, xen_pfn_t mfn_list[])
@@ -1220,6 +1223,7 @@ static int acquire_vmtrace_buf(
 
     return nr_frames;
 }
+#endif
 
 /*
  * Returns -errno on error, or positive in the range [1, nr_frames] on
@@ -1238,8 +1242,10 @@ static int _acquire_resource(
     case XENMEM_resource_ioreq_server:
         return acquire_ioreq_server(d, id, frame, nr_frames, mfn_list);
 
+#ifdef CONFIG_VMTRACE
     case XENMEM_resource_vmtrace_buf:
         return acquire_vmtrace_buf(d, id, frame, nr_frames, mfn_list);
+#endif
 
     default:
         ASSERT_UNREACHABLE();
diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
index b2787c010890..cf0258223f50 100644
--- a/xen/common/vm_event.c
+++ b/xen/common/vm_event.c
@@ -441,7 +441,8 @@ static int vm_event_resume(struct domain *d, struct vm_event_domain *ved)
             if ( rsp.flags & VM_EVENT_FLAG_GET_NEXT_INTERRUPT )
                 vm_event_monitor_next_interrupt(v);
 
-            if ( rsp.flags & VM_EVENT_FLAG_RESET_VMTRACE )
+            if ( IS_ENABLED(CONFIG_VMTRACE) &&
+                 (rsp.flags & VM_EVENT_FLAG_RESET_VMTRACE) )
                 vm_event_reset_vmtrace(v);
 
             if ( rsp.flags & VM_EVENT_FLAG_VCPU_PAUSED )
diff --git a/xen/include/xen/domain.h b/xen/include/xen/domain.h
index 8aab05ae93c8..aa86a9f46022 100644
--- a/xen/include/xen/domain.h
+++ b/xen/include/xen/domain.h
@@ -191,7 +191,11 @@ void vnuma_destroy(struct vnuma_info *vnuma);
 static inline void vnuma_destroy(struct vnuma_info *vnuma) { ASSERT(!vnuma); }
 #endif
 
+#ifdef CONFIG_VMTRACE
 extern bool vmtrace_available;
+#else
+#define vmtrace_available false
+#endif
 
 extern bool vpmu_is_available;
 
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 02bdc256ce37..dcd8647e0d36 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -300,9 +300,11 @@ struct vcpu
     /* vPCI per-vCPU area, used to store data for long running operations. */
     struct vpci_vcpu vpci;
 
+#ifdef CONFIG_VMTRACE
     struct {
         struct page_info *pg; /* One contiguous allocation of d->vmtrace_size */
     } vmtrace;
+#endif
 
     struct arch_vcpu arch;
 
@@ -623,7 +625,9 @@ struct domain
     unsigned int nr_altp2m;    /* Number of altp2m tables. */
 #endif
 
+#ifdef CONFIG_VMTRACE
     unsigned int vmtrace_size; /* Buffer size in bytes, or 0 to disable. */
+#endif
 
 #ifdef CONFIG_ARGO
     /* Argo interdomain communication support */
-- 
2.34.1

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.

> --- 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.

Jan

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

On 18.11.2025 13:42, Grygorii Strashko wrote:
> 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>

Any place where only spaces are used is malformed. "help" text with its special
indentation is a separate thing, of course.

>>> --- 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;

Almost, albeit the off-by-1 indentation may also merely be an effect of your mailer.

Jan