From: Grygorii Strashko <grygorii_strashko@epam.com>

Since commit b99227347230 ("x86: Fix AMD_SVM and INTEL_VMX dependency") the
HVM Intel VT-x support can be disabled, but it still keeps VMX
code partially built-in. Particularly in HVM code there are two places:

1) hvm/dom0_build.c
 dom0_construct_pvh()->pvh_populate_p2m()
    ...
    if ( cpu_has_vmx && paging_mode_hap(d) && !vmx_unrestricted_guest(v) )
    {
        ...
        [unreachable for !cpu_has_vmx case]
        rc = pvh_setup_vmx_realmode_helpers(d);

pvh_setup_vmx_realmode_helpers() allocates memory and configures
 HVM_PARAM_VM86_TSS_SIZED
 HVM_PARAM_IDENT_PT

2) hvm/hvm.c
 hvm_set_param()
    ...
    case HVM_PARAM_IDENT_PT:

        if ( !paging_mode_hap(d) || !cpu_has_vmx )
        {
            d->arch.hvm.params[index] = value;
            break;
        }
        [unreachable for !cpu_has_vmx case]
        ...

Hence HVM_PARAM_IDENT_PT/HVM_PARAM_VM86_TSS_SIZED are used only by VMX code
above code become unreachable in !cpu_has_vmx case and can be optimazed
when !CONFIG_INTEL_VMX.

Replace "cpu_has_vmx" with using_vmx() to account !CONFIG_INTEL_VMX and allow
compiler DCE to optimize code.

Signed-off-by: Grygorii Strashko <grygorii_strashko@epam.com>
---
add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-604 (-604)
Function                                     old     new   delta
hvm_set_param                               1602    1473    -129
dom0_construct_pvh                          4438    3963    -475
Total: Before=3567191, After=3566587, chg -0.02%

 xen/arch/x86/hvm/dom0_build.c | 2 +-
 xen/arch/x86/hvm/hvm.c        | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/dom0_build.c b/xen/arch/x86/hvm/dom0_build.c
index 5551f9044836..5ac2cf8394d8 100644
--- a/xen/arch/x86/hvm/dom0_build.c
+++ b/xen/arch/x86/hvm/dom0_build.c
@@ -473,7 +473,7 @@ static int __init pvh_populate_p2m(struct domain *d)
         }
     }
 
-    if ( cpu_has_vmx && paging_mode_hap(d) && !vmx_unrestricted_guest(v) )
+    if ( using_vmx() && paging_mode_hap(d) && !vmx_unrestricted_guest(v) )
     {
         /*
          * Since Dom0 cannot be migrated, we will only setup the
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 95a80369b9b8..70331aeec9a0 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4302,7 +4302,7 @@ static int hvm_set_param(struct domain *d, uint32_t index, uint64_t value)
          * Only actually required for VT-x lacking unrestricted_guest
          * capabilities.  Short circuit the pause if possible.
          */
-        if ( !paging_mode_hap(d) || !cpu_has_vmx )
+        if ( !paging_mode_hap(d) || !using_vmx() )
         {
             d->arch.hvm.params[index] = value;
             break;
-- 
2.34.1

On Wed Sep 24, 2025 at 12:14 PM CEST, Grygorii Strashko wrote:
> From: Grygorii Strashko <grygorii_strashko@epam.com>
>
> Since commit b99227347230 ("x86: Fix AMD_SVM and INTEL_VMX dependency") the
> HVM Intel VT-x support can be disabled, but it still keeps VMX
> code partially built-in. Particularly in HVM code there are two places:
>
> 1) hvm/dom0_build.c
>  dom0_construct_pvh()->pvh_populate_p2m()
>     ...
>     if ( cpu_has_vmx && paging_mode_hap(d) && !vmx_unrestricted_guest(v) )
>     {
>         ...
>         [unreachable for !cpu_has_vmx case]
>         rc = pvh_setup_vmx_realmode_helpers(d);
>
> pvh_setup_vmx_realmode_helpers() allocates memory and configures
>  HVM_PARAM_VM86_TSS_SIZED
>  HVM_PARAM_IDENT_PT
>
> 2) hvm/hvm.c
>  hvm_set_param()
>     ...
>     case HVM_PARAM_IDENT_PT:
>
>         if ( !paging_mode_hap(d) || !cpu_has_vmx )
>         {
>             d->arch.hvm.params[index] = value;
>             break;
>         }
>         [unreachable for !cpu_has_vmx case]
>         ...

nit: These (1) and (2) are rather large for a commit message. I wouldn't mind
if they went away and the rest of the commit message was adjusted to make it
a bit leaner.

Either way, with or without this change...

>
> Hence HVM_PARAM_IDENT_PT/HVM_PARAM_VM86_TSS_SIZED are used only by VMX code
> above code become unreachable in !cpu_has_vmx case and can be optimazed
> when !CONFIG_INTEL_VMX.
>
> Replace "cpu_has_vmx" with using_vmx() to account !CONFIG_INTEL_VMX and allow
> compiler DCE to optimize code.
>
> Signed-off-by: Grygorii Strashko <grygorii_strashko@epam.com>

... Reviewed-by: Alejandro Vallejo <alejandro.garciavallejo@amd.com>

Cheers,
Alejandro