Re: [XEN][PATCH] x86/hvm: revise "cpu_has_vmx" usage for !CONFIG_INTEL_VMX case

[Grygorii Strashko <grygorii_strashko@xxxxxxxx>]

Hi Jan,

On 13.11.25 09:31, Jan Beulich wrote:
> On 12.11.2025 21:25, Grygorii Strashko wrote:
> > Could it be merged if no other comments, please?
> It's on my list of things to put in. However, since you ask: You do realize that
> all your recent postings were while the tree was frozen? That's okay-ish, but
> when preparing a release people would preferably focus on the release, not on
> what is to come afterwards. This change of yours, like quite a few from others,
> will need to wait until the tree is fully open again. It having been progressed
> to the point where it can in principle go in was already a courtesy, considering
> the timing.
I'm very sorry for disturbing you and being annoying in this regards.

> > On 24.09.25 13:14, 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>
> > > ---
> > > 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;
--
Best regards,
-grygorii