Re: [PATCH 1/3] x86/amd: Fold another DE_CFG edit into amd_init_de_cfg()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.12.2025 11:57, Andrew Cooper wrote:
> Fam12h processors aren't SMT-capable so there are no race condition worries
> with this path.  Nevertheless, group it together with the other DE_CFG
> modifications.

With this, ...

> Fixes: d0c75dc4c028 ("x86/amd: Fix race editing DE_CFG")

... isn't this more like Amends:? Aiui this wouldn't need backporting.

> --- a/xen/arch/x86/cpu/amd.c
> +++ b/xen/arch/x86/cpu/amd.c
> @@ -920,6 +920,13 @@ void amd_init_de_cfg(const struct cpuinfo_x86 *c)
>      if ( zenbleed_use_chickenbit() )
>          new |= (1 << 9);
>  
> +    /*
> +     * Erratum #665, doc 44739.  Integer divide instructions may cause
> +     * unpredictable behaviour.
> +     */
> +    if ( c->family == 0x12 )
> +        new |= 1U << 31;
> +
>      /* Avoid reading DE_CFG if we don't intend to change anything. */
>      if ( !new )
>          return;
> @@ -1201,15 +1208,6 @@ static void cf_check init_amd(struct cpuinfo_x86 *c)
>                                           smp_processor_id());
>                       wrmsrl(MSR_AMD64_LS_CFG, value | (1 << 15));
>               }
> -     } else if (c->x86 == 0x12) {
> -             rdmsrl(MSR_AMD64_DE_CFG, value);
> -             if (!(value & (1U << 31))) {
> -                     if (c == &boot_cpu_data || opt_cpu_info)
> -                             printk_once(XENLOG_WARNING
> -                                         "CPU%u: Applying workaround for 
> erratum 665\n",
> -                                         smp_processor_id());
> -                     wrmsrl(MSR_AMD64_DE_CFG, value | (1U << 31));
> -             }
>       }

Are you deliberately getting rid of the log message?

And I assume it is deliberate that the adjustment no longer is done when we're
running virtualized ourselves?

Both imo want making explicit in the description.

Jan