Re: [PATCH v2 2/3] x86/spec: fix BRANCH_HARDEN option to only be set when build-enabled

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.02.2024 13:06, Roger Pau Monne wrote:
> The current logic to handle the BRANCH_HARDEN option will report it as enabled
> even when build-time disabled. Fix this by only allowing the option to be set
> when support for it is built into Xen.
> 
> Fixes: 2d6f36daa086 ('x86/nospec: Introduce CONFIG_SPECULATIVE_HARDEN_BRANCH')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Changes since v1:
>  - Use no_config_param().
> ---
>  xen/arch/x86/spec_ctrl.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
> index 421fe3f640df..4ce8a7a0b8ef 100644
> --- a/xen/arch/x86/spec_ctrl.c
> +++ b/xen/arch/x86/spec_ctrl.c
> @@ -50,7 +50,8 @@ static int8_t __initdata opt_psfd = -1;
>  int8_t __ro_after_init opt_ibpb_ctxt_switch = -1;
>  int8_t __read_mostly opt_eager_fpu = -1;
>  int8_t __read_mostly opt_l1d_flush = -1;
> -static bool __initdata opt_branch_harden = true;
> +static bool __initdata opt_branch_harden =
> +    IS_ENABLED(CONFIG_SPECULATIVE_HARDEN_BRANCH);
>  
>  bool __initdata bsp_delay_spec_ctrl;
>  uint8_t __read_mostly default_xen_spec_ctrl;
> @@ -268,7 +269,14 @@ static int __init cf_check parse_spec_ctrl(const char *s)
>          else if ( (val = parse_boolean("l1d-flush", s, ss)) >= 0 )
>              opt_l1d_flush = val;
>          else if ( (val = parse_boolean("branch-harden", s, ss)) >= 0 )
> +        {
> +#ifdef CONFIG_SPECULATIVE_HARDEN_BRANCH
>              opt_branch_harden = val;
> +#else
> +            no_config_param("SPECULATIVE_HARDEN_BRANCH", "spec-ctrl", s, ss);
> +            rc = -EINVAL;
> +#endif
> +        }

If you use #ifdef (rather than IS_ENABLED()) here, the variable probably
shouldn't exist at all when the Kconfig option is off (albeit yet, another
#ifdef would then be needed higher up in the function). Question is - why
don't you use IS_ENABLED() here as well?

Jan