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
>
>> +#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.
>
>> 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.
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.
Cheers,
Alejandro
