Re: [PATCH 1/2] x86: Make hvm_enabled a constant unless both PV and HVM are both compiled

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Feb 13, 2026 at 06:04:33PM +0100, Alejandro Vallejo wrote:
> On Fri Feb 13, 2026 at 3:26 PM CET, Roger Pau Monné wrote:
> > On Fri, Feb 13, 2026 at 02:37:29PM +0100, Alejandro Vallejo wrote:
> >> PV-shim already has hvm_enabled optimised to false, but there's no
> >> reason HVM-only builds can't benefit from the same optimisation as long
> >> as we add a boot-time panic in case HVM support is missed. Many branches
> >> go away, some in the potentially hot switch_cr3_cr4.
> >> 
> >> HVM-only:
> >>   add/remove: 0/1 grow/shrink: 1/9 up/down: 1/-162 (-161)
> >>   Function                                     old     new   delta
> >>   arch_do_physinfo                              79      80      +1
> >>   hvm_enabled                                    1       -      -1
> >>   symbols_offsets                            30732   30728      -4
> >>   symbols_names                             108029  108022      -7
> >>   symbols_sorted_offsets                     60656   60648      -8
> >>   flush_area_local                             571     562      -9
> >>   switch_cr3_cr4                               311     300     -11
> >>   init_xen_cap_info                             62      43     -19
> >>   arch_sanitise_domain_config                  885     863     -22
> >>   init_guest_cpu_policies                     1270    1247     -23
> >>   hvm_domain_initialise                       1127    1069     -58
> >>   Total: Before=3797004, After=3796843, chg -0.00%
> >> 
> >> With hvm_enabled const-ified, it's fine to take hvm_flush_guest_tlbs()
> >> outside the CONFIG_HVM ifdef and remove the stub. They compile to the
> >> same code after DCE.
> >> 
> >> Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
> >> ---
> >>  xen/arch/x86/hvm/hvm.c             |  9 +++++++++
> >>  xen/arch/x86/include/asm/hvm/hvm.h | 30 +++++++++++++++---------------
> >>  2 files changed, 24 insertions(+), 15 deletions(-)
> >> 
> >> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> >> index 4d37a93c57..da56944e74 100644
> >> --- a/xen/arch/x86/hvm/hvm.c
> >> +++ b/xen/arch/x86/hvm/hvm.c
> >> @@ -72,7 +72,9 @@
> >>  
> >>  #include <compat/hvm/hvm_op.h>
> >>  
> >> +#ifdef CONFIG_PV
> >>  bool __read_mostly hvm_enabled;
> >
> > __ro_after_init?
> 
> Yeah, seems to have been missing originally

I bet __ro_after_init wasn't available when this was introduced.

> >
> >> +#endif /* CONFIG_PV */
> >>  
> >>  #ifdef DBG_LEVEL_0
> >>  unsigned int opt_hvm_debug_level __read_mostly;
> >> @@ -173,9 +175,16 @@ static int __init cf_check hvm_enable(void)
> >>          svm_fill_funcs();
> >>  
> >>      if ( fns == NULL )
> >> +    {
> >> +        if ( !IS_ENABLED(CONFIG_PV) )
> >> +            panic("HVM support not detected and PV compiled-out\n");
> >> +
> >>          return 0;
> >> +    }
> >>  
> >> +#ifdef CONFIG_PV
> >
> > CONFIG_HVM I think?
> 
> No. CONFIG_HVM always holds here, what we want to remove is hvm_enabled being
> present when CONFIG_PV is _also_ present.

Yeah, Andrew already noticed that.

> >
> >>      hvm_enabled = 1;
> >
> > = true;
> 
> True enough.
> 
> >
> >> +#endif /* CONFIG_PV */
> >>  
> >>      printk("HVM: %s enabled\n", fns->name);
> >>      if ( !hap_supported(&hvm_funcs) )
> >> diff --git a/xen/arch/x86/include/asm/hvm/hvm.h 
> >> b/xen/arch/x86/include/asm/hvm/hvm.h
> >> index 7d9774df59..dc609bf4cb 100644
> >> --- a/xen/arch/x86/include/asm/hvm/hvm.h
> >> +++ b/xen/arch/x86/include/asm/hvm/hvm.h
> >> @@ -261,7 +261,11 @@ struct hvm_function_table {
> >>  };
> >>  
> >>  extern struct hvm_function_table hvm_funcs;
> >> +#if defined(CONFIG_PV) && defined(CONFIG_HVM)
> >>  extern bool hvm_enabled;
> >> +#else
> >> +#define hvm_enabled IS_ENABLED(CONFIG_HVM)
> >> +#endif
> >>  extern int8_t hvm_port80_allowed;
> >>  
> >>  extern const struct hvm_function_table *start_svm(void);
> >> @@ -399,6 +403,17 @@ static inline bool using_svm(void)
> >>  #define hvm_is_in_uc_mode(d) \
> >>      (using_vmx() && (d)->arch.hvm.vmx.in_uc_mode)
> >>  
> >> +/*
> >> + * Called to ensure than all guest-specific mappings in a tagged TLB are
> >> + * flushed; does *not* flush Xen's TLB entries, and on processors without 
> >> a
> >> + * tagged TLB it will be a noop.
> >> + */
> >> +static inline void hvm_flush_guest_tlbs(void)
> >> +{
> >> +    if ( hvm_enabled )
> >> +        hvm_asid_flush_core();
> >> +}
> >> +
> >>  #ifdef CONFIG_HVM
> >>  
> >>  #define hvm_get_guest_tsc(v) hvm_get_guest_tsc_fixed(v, 0)
> >> @@ -498,17 +513,6 @@ static inline void hvm_set_tsc_offset(struct vcpu *v, 
> >> uint64_t offset)
> >>      alternative_vcall(hvm_funcs.set_tsc_offset, v, offset);
> >>  }
> >>  
> >> -/*
> >> - * Called to ensure than all guest-specific mappings in a tagged TLB are 
> >> - * flushed; does *not* flush Xen's TLB entries, and on processors without 
> >> a 
> >> - * tagged TLB it will be a noop.
> >> - */
> >> -static inline void hvm_flush_guest_tlbs(void)
> >> -{
> >> -    if ( hvm_enabled )
> >> -        hvm_asid_flush_core();
> >> -}
> >
> > Is there any specific reason you only pick hvm_flush_guest_tlbs().
> 
> I didn't try to remove more. That one was the only one with hvm_enabled in the
> static inline so it seems easy to pick apart.

Oh, I see.  That one already had the hvm_enabled condition in it's
non-stub version.

> I just tried compiling and I require very few additions to make the build
> compile without stubs. I'll send another version with the adjustments needed.

OK, I think if we could unify them that would make the header smaller
and easier to read.

Thanks, Roger.