Re: [PATCH v2 for-4.21 1/2] x86/AMD: disable RDSEED on Fam17 model 47 stepping 0

["Teddy Astie" <teddy.astie@xxxxxxxxxx>]

Le 28/10/2025 à 16:35, Jan Beulich a écrit :
> This particular variant has an error that causes RDSEED to always return
> 0xffffffff, while RDRAND works correctly.
>
> Inspired by Linux commit 5b937a1ed64ebeba8876e398110a5790ad77407c
> ("x86/rdrand: Disable RDSEED on AMD Cyan Skillfish").
>
> Like for RDRAND, permit a command line override to be used to keep
> RDSEED enabled.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Considering how it is described, I didn't think probing RDSEED (like we
> do for RDRAND) would be necessary.
>
> Am I going too far in also updating cpuidmask_defaults here, or is us
> not doing so for the RDRAND disabling actually an oversight?
>
> Using warning_add() may not be quite appropriate, as we don't really
> mean the admin to possibly override this with "cpuid=rdseed" (that's
> only a last resort, in case the issue is yet more limited in scope). But
> mere printk() would feel like hiding the information in the middle of
> lots of other output.
> ---
> v2: Correctly check model, not (again) family.
>
> --- a/xen/arch/x86/cpu/amd.c
> +++ b/xen/arch/x86/cpu/amd.c
> @@ -1219,6 +1219,24 @@ static void cf_check init_amd(struct cpu
>               }
>               break;
>
> +     case 0x17:
> +             /*
> +              * Fam17 model 47 stepping 0 has an error that causes RDSEED to
> +              * always return 0xffffffff (while RDRAND works correctly).
> +              */
> +             if (c == &boot_cpu_data &&
> +                 c->model == 0x47 && c->stepping == 0 &&
> +                 cpu_has(c, X86_FEATURE_RDSEED) &&
> +                 !is_forced_cpu_cap(X86_FEATURE_RDSEED)) {
> +                     static const char __initconst text[] =
> +                             "RDSEED is unreliable on this hardware; 
> disabling its exposure\n";
> +
> +                     setup_clear_cpu_cap(X86_FEATURE_RDSEED);
> +                     cpuidmask_defaults._7ab0 &= 
> ~cpufeat_mask(X86_FEATURE_RDSEED);
> +                     warning_add(text);
> +             }
> +             break;
> +
>       case 0x19:
>               /*
>                * Zen3 (Fam19h model < 0x10) parts are not susceptible to
> --- a/xen/arch/x86/cpu-policy.c
> +++ b/xen/arch/x86/cpu-policy.c
> @@ -123,6 +123,10 @@ static void __init cf_check _parse_xen_c
>       else if ( feat == X86_FEATURE_RDRAND &&
>                 (cpuid_ecx(1) & cpufeat_mask(X86_FEATURE_RDRAND)) )
>           setup_force_cpu_cap(X86_FEATURE_RDRAND);
> +    else if ( feat == X86_FEATURE_RDSEED &&
> +              cpuid_eax(0) >= 7 &&
> +              (cpuid_count_ebx(7, 0) & cpufeat_mask(X86_FEATURE_RDSEED)) )
> +        setup_force_cpu_cap(X86_FEATURE_RDSEED);
>   }
>
>   static int __init cf_check parse_xen_cpuid(const char *s)
>
>

Reviewed-by: Teddy Astie <teddy.astie@xxxxxxxxxx>


--
Teddy Astie | Vates XCP-ng Developer

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