Re: [tip: x86/urgent] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

[Sasha Levin]

On Fri, Aug 23, 2019 at 01:10:49AM -0000, tip-bot2 for Tom Lendacky wrote:
> The following commit has been merged into the x86/urgent branch of tip:
>
> Commit-ID:     c49a0a80137c7ca7d6ced4c812c9e07a949f6f24
> Gitweb:        https://git.kernel.org/tip/c49a0a80137c7ca7d6ced4c812c9e07a949f6f24
> Author:        Tom Lendacky <thomas.lendacky@xxxxxxx>
> AuthorDate:    Mon, 19 Aug 2019 15:52:35
> Committer:     Borislav Petkov <bp@xxxxxxx>
> CommitterDate: Mon, 19 Aug 2019 19:42:52 +02:00
>
> x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h
>
> There have been reports of RDRAND issues after resuming from suspend on
> some AMD family 15h and family 16h systems. This issue stems from a BIOS
> not performing the proper steps during resume to ensure RDRAND continues
> to function properly.
>
> RDRAND support is indicated by CPUID Fn00000001_ECX[30]. This bit can be
> reset by clearing MSR C001_1004[62]. Any software that checks for RDRAND
> support using CPUID, including the kernel, will believe that RDRAND is
> not supported.
>
> Update the CPU initialization to clear the RDRAND CPUID bit for any family
> 15h and 16h processor that supports RDRAND. If it is known that the family
> 15h or family 16h system does not have an RDRAND resume issue or that the
> system will not be placed in suspend, the "rdrand=force" kernel parameter
> can be used to stop the clearing of the RDRAND CPUID bit.
>
> Additionally, update the suspend and resume path to save and restore the
> MSR C001_1004 value to ensure that the RDRAND CPUID setting remains in
> place after resuming from suspend.
>
> Note, that clearing the RDRAND CPUID bit does not prevent a processor
> that normally supports the RDRAND instruction from executing it. So any
> code that determined the support based on family and model won't #UD.
>
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Chen Yu <yu.c.chen@xxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: "linux-doc@xxxxxxxxxxxxxxx" <linux-doc@xxxxxxxxxxxxxxx>
> Cc: "linux-pm@xxxxxxxxxxxxxxx" <linux-pm@xxxxxxxxxxxxxxx>
> Cc: Nathan Chancellor <natechancellor@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Pavel Machek <pavel@xxxxxx>
> Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: "x86@xxxxxxxxxx" <x86@xxxxxxxxxx>
> Link: https://lkml.kernel.org/r/7543af91666f491547bd86cebb1e17c66824ab9f.1566229943.git.thomas.lendacky@xxxxxxx
> ---
> Documentation/admin-guide/kernel-parameters.txt |  7 +-
> arch/x86/include/asm/msr-index.h                |  1 +-
> arch/x86/kernel/cpu/amd.c                       | 66 +------------
> arch/x86/power/cpu.c                            | 86 ++--------------
> 4 files changed, 13 insertions(+), 147 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 4c19719..47d981a 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4090,13 +4090,6 @@
> 			Run specified binary instead of /init from the ramdisk,
> 			used for early userspace startup. See initrd.
>
> -	rdrand=		[X86]
> -			force - Override the decision by the kernel to hide the
> -				advertisement of RDRAND support (this affects
> -				certain AMD processors because of buggy BIOS
> -				support, specifically around the suspend/resume
> -				path).
> -

Why is this being removed (along with supporting code)?

--
Thanks,
Sasha