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Re: [XEN][PATCH] x86/hvm: revise "cpu_has_vmx" usage for !CONFIG_INTEL_VMX case
- To: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 8 Oct 2025 17:53:20 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 08 Oct 2025 15:53:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 24.09.2025 12:14, Grygorii Strashko wrote:
> From: Grygorii Strashko <grygorii_strashko@xxxxxxxx>
>
> 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@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
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