Re: [PATCH v2 12/23] x86/fsgsbase: Make gskern accesses safe under FRED

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.08.2025 17:03, Andrew Cooper wrote:
> In principle, the following can also be used for read_registers()
> 
>     diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
>     index 5799770a2f71..0b0fdf2c5ac4 100644
>     --- a/xen/arch/x86/traps.c
>     +++ b/xen/arch/x86/traps.c
>     @@ -125,16 +125,21 @@ static void read_registers(struct extra_state 
> *state)
>          state->cr3 = read_cr3();
>          state->cr4 = read_cr4();
> 
>     -    if ( !(state->cr4 & X86_CR4_FRED) && (state->cr4 & X86_CR4_FSGSBASE) 
> )
>     +    if ( state->cr4 & X86_CR4_FSGSBASE )
>          {
>              state->fsb = __rdfsbase();
>              state->gsb = __rdgsbase();
>     +
>     +        if ( state->cr4 & X86_CR4_FRED )
>     +            goto gskern_fred;
>     +
>              state->gss = __rdgskern();

I'm irritated by this patch context here vs ...

> --- a/xen/arch/x86/include/asm/fsgsbase.h
> +++ b/xen/arch/x86/include/asm/fsgsbase.h
> @@ -79,7 +79,9 @@ static inline unsigned long read_gs_base(void)
>  
>  static inline unsigned long read_gs_shadow(void)
>  {
> -    if ( read_cr4() & X86_CR4_FSGSBASE )
> +    unsigned long cr4 = read_cr4();
> +
> +    if ( !(cr4 & X86_CR4_FRED) && (cr4 & X86_CR4_FSGSBASE) )
>          return __rdgs_shadow();

... the one here. Was the former (and the subject of the patch) not updated
yet (kern => shadow)? On the assumption that that's what has happened, and
hence preferably with the subject also adjusted
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

As to the alternative, I in particular don't overly like the goto there. I
would consider that an option only if in turn a simplification elsewhere
resulted.

Jan