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.

Fixes: d0c75dc4c028 ("x86/amd: Fix race editing DE_CFG")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>

I apparently couldn't count how many examples we had editing DE_CFG...
---
 xen/arch/x86/cpu/amd.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index 47c109f89804..4dc9157836ad 100644
--- 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));
-		}
 	}
 
 	/* AMD CPUs do not support SYSENTER outside of legacy mode. */
-- 
2.39.5

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