Re: [PATCH V3 (resend) 06/19] x86: Add a boot option to enable and disable the direct map

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.05.2024 15:40, Elias El Yandouzi wrote:
> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -80,12 +80,29 @@ config HAS_PMAP
>  config HAS_SCHED_GRANULARITY
>       bool
>  
> +config HAS_SECRET_HIDING
> +     bool
> +
>  config HAS_UBSAN
>       bool
>  
>  config MEM_ACCESS_ALWAYS_ON
>       bool
>  
> +config SECRET_HIDING
> +    bool "Secret hiding"
> +    depends on HAS_SECRET_HIDING
> +    help
> +             The directmap contains mapping for most of the RAM which makes 
> domain
> +             memory easily accessible. While making the performance better, 
> it also makes
> +             the hypervisor more vulnerable to speculation attacks.
> +
> +             Enabling this feature will allow the user to decide whether the 
> memory
> +             is always mapped at boot or mapped only on demand (see the 
> command line
> +             option "directmap").
> +
> +             If unsure, say N.
> +
>  config MEM_ACCESS
>       def_bool MEM_ACCESS_ALWAYS_ON
>       prompt "Memory Access and VM events" if !MEM_ACCESS_ALWAYS_ON

Surely there's a better place to add this new setting than between two
dependent options (MEM_ACCESS_ALWAYS_ON and MEM_ACCESS).

> --- a/xen/include/xen/mm.h
> +++ b/xen/include/xen/mm.h
> @@ -167,6 +167,13 @@ extern unsigned long max_page;
>  extern unsigned long total_pages;
>  extern paddr_t mem_hotplug;
>  
> +extern bool opt_directmap;
> +
> +static inline bool has_directmap(void)
> +{
> +    return opt_directmap;
> +}

As indicated before, with the Kconfig setting off I think we want to
have an alternative

#define opt_directmap true

There's no need to impact generated code by needing to look at a "variable"
which is never going to change value.

Jan