linux-kernel - Re: [v2 1/3] x86/microcode/intel: Check microcode revision before updating sibling threads

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Date:   Thu, 22 Feb 2018 12:00:57 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Ashok Raj <ashok.raj@...el.com>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [v2 1/3] x86/microcode/intel: Check microcode revision before
 updating sibling threads

On Wed, Feb 21, 2018 at 10:33:23PM -0800, Ashok Raj wrote:
> After updating microcode on one of the threads in the core, the
> thread sibling automatically gets the update since the microcode
> resources are shared. Check the ucode revision on the CPU before
> performing a ucode update.
> 
> Signed-off-by: Ashok Raj <ashok.raj@...el.com>
> Cc: X86 ML <x86@...nel.org>
> Cc: LKML <linux-kernel@...r.kernel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Tony Luck <tony.luck@...el.com>
> Cc: Andi Kleen <andi.kleen@...el.com>
> Cc: Boris Petkov <bp@...e.de>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Arjan Van De Ven <arjan.van.de.ven@...el.com>
> 
> Updates:
> v2: Address Boris's to cleanup apply_microcode_intel
> v3: Fixups per Ingo: Spell Checks

That changelog...

> ---

... comes under this line so that it can be ignored by tools.

>  arch/x86/kernel/cpu/microcode/intel.c | 28 +++++++++++++++++++++++++---
>  1 file changed, 25 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/microcode/intel.c b/arch/x86/kernel/cpu/microcode/intel.c
> index 09b95a7..137c9f5 100644
> --- a/arch/x86/kernel/cpu/microcode/intel.c
> +++ b/arch/x86/kernel/cpu/microcode/intel.c
> @@ -589,6 +589,18 @@ static int apply_microcode_early(struct ucode_cpu_info *uci, bool early)
>  	if (!mc)
>  		return 0;
>  
> +	rev = intel_get_microcode_revision();

Ok, so I'm still wondering what this patch is trying to achieve.

intel_get_microcode_revision() does already *two* MSR reads and a CPUID.
So it is not speed improvements - it actually makes loading slower due to that
checking.

So why are we doing this again?

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--