linux-kernel - Re: [PATCH] x86/CPU/AMD: Only apply Zenbleed fix for Zen2 during late microcode load

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Message-ID: <ZvGPqWckmaEyYBz4@AUSJOHALLEN.amd.com>
Date: Mon, 23 Sep 2024 10:56:25 -0500
From: John Allen <john.allen@....com>
To: bp@...en8.de, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] x86/CPU/AMD: Only apply Zenbleed fix for Zen2 during
 late microcode load

On Mon, Sep 23, 2024 at 03:41:12PM +0000, John Allen wrote:
> A problem was introduced with f69759b ("x86/CPU/AMD: Move Zenbleed check to
> the Zen2 init function") where a bit in the DE_CFG MSR is getting set after
> a microcode late load.
> 
> The problem is that the microcode late load path calls into
> amd_check_microcode and subsequently zen2_zenbleed_check. Since the patch
> removes the cpu_has_amd_erratum check from zen2_zenbleed_check, this will
> cause all non-Zen2 cpus to go through the function and set the bit in the
> DE_CFG MSR.
> 
> Call into the zenbleed fix path on Zen2 cpus only.
> 
> Fixes: f69759be251d ("x86/CPU/AMD: Move Zenbleed check to the Zen2 init function")

Should probably go into stable as well.

Cc: <stable@...r.kernel.org>

> Signed-off-by: John Allen <john.allen@....com>
> ---
>  arch/x86/kernel/cpu/amd.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 015971adadfc..368344e1394b 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -1202,5 +1202,6 @@ void amd_check_microcode(void)
>  	if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD)
>  		return;
>  
> -	on_each_cpu(zenbleed_check_cpu, NULL, 1);
> +	if (boot_cpu_has(X86_FEATURE_ZEN2))
> +		on_each_cpu(zenbleed_check_cpu, NULL, 1);
>  }
> -- 
> 2.34.1
>