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 <[email protected]>
> >> ---
> >> 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.
