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 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?

> +#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?

>      hvm_enabled = 1;

= true;

> +#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().
Given what you do with hvm_enabled you could probably generalize all
the dummy helpers in the !CONFIG_HVM section now?

Thanks, Roger.