Re: [PATCH] x86/pm: Save AMD CPUID leaf 7 across S3

[Borislav Petkov]

+ Mario.

On Tue, Aug 04, 2026 at 11:13:41AM +0100, Sean Rhodes wrote:
> Some AMD systems return from S3 with the boot CPU CPUID leaf 7
> override MSR reset. On Byte/Cezanne, CPUID leaf 7 EBX becomes
> 0x219c9fb9 on CPU0 after S3 even though XTEST still raises #UD.
> Userspace can then select RTM-optimised glibc paths and fault after
> resume.

Do you have the latest BIOS on that machine?

> The x86 suspend code already has a small MSR save/restore list for
> firmware-modified MSRs and restores it after resume microcode has been
> loaded. Add MSR_AMD64_CPUID_FN_7 for the AMD families that can modify
> this MSR during CPU init so the pre-suspend CPUID mask is restored
> before tasks are thawed.
> 
> This covers family 17h, 19h, and 1Ah:

Why?

> family 17h has the Cyan Skillfish
> RDSEED leaf-7 quirk, family 19h covers the observed Cezanne issue, and
> family 1Ah has the Zen5 RDSEED leaf-7 quirk.

Those RDSEED quirks need more details as to why they need to be applied here.

> Tested on StarBook Mk V (Byte/Cezanne, 26.07 firmware):
> - without this restore, dmesg showed "ACPI: PM: Low-level resume complete"
>   and "PM: suspend exit"; the post-S3 CPUID probe showed CPU0 leaf 7 EBX
>   0x219c9fb9 with hle=1 rtm=1 while XTEST still raised #UD.
> - a temporary PM resume hook clearing the same MSR logged
>   "CPU0 MSR_AMD64_CPUID_FN_7 0x219c9fb9 -> 0x219c97a9"; the post-S3 probe
>   showed hle=0 rtm=0 on all CPUs.
> - this patch booted on Byte and registered the MSR save path; dmesg showed
>   "x86/pm: AMD family 0x19 CPU detected"; pm_test freezer/devices/platform/
>   processors/core passed with the same boot ID.

For future patches: testing goes...

> 
> Tested-by: Sean Rhodes <sean@starlabs.systems>
> Signed-off-by: Sean Rhodes <sean@starlabs.systems>
> ---

<-- here, under that line.

>  arch/x86/power/cpu.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> index 702f30eaf9c4..b85061f4cee4 100644
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -467,9 +467,24 @@ static int msr_save_cpuid_features(const struct x86_cpu_id *c)
>  	return msr_build_context(cpuid_msr_id, ARRAY_SIZE(cpuid_msr_id));
>  }
>  
> +static int msr_save_amd_leaf7_cpuid_features(const struct x86_cpu_id *c)
> +{
> +	u32 cpuid_msr_id[] = {
> +		MSR_AMD64_CPUID_FN_7,
> +	};
> +
> +	pr_info("x86/pm: AMD family %#x CPU detected, saving CPUID leaf 7 MSR across suspend.\n",

I guess pr_info_once() is fine here.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette