linux-kernel - Re: [Patch V1 4/7] x86/microcode/core: Take a snapshot before and after applying microcode

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Message-ID: <Y5D20qLOmrj4d2w4@zn.tnic>
Date:   Wed, 7 Dec 2022 21:25:54 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Ashok Raj <ashok.raj@...el.com>
Cc:     X86-kernel <x86@...nel.org>,
        LKML Mailing List <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>, alison.schofield@...el.com,
        reinette.chatre@...el.com
Subject: Re: [Patch V1 4/7] x86/microcode/core: Take a snapshot before and
 after applying microcode

On Tue, Nov 29, 2022 at 01:08:29PM -0800, Ashok Raj wrote:
> The kernel caches features about each CPU's features at boot in an
> x86_capability[] structure. The microcode update takes one snapshot and
> compares it with the saved copy at boot.
> 
> However, the capabilities in the boot copy can be turned off as a result of
> certain command line parameters or configuration restrictions. This can
> cause a mismatch when comparing the values before and after the microcode
> update.

Hmm, but if that has happened, the capabilities will be turned off in
your @orig argument below?

Or are you saying that this copy_cpu_caps() read before the update will
overwrite the cleared bits with the their actual values from CPUID so
that what you really wanna compare here is *hardware* CPUID bits before
and after and not our potentially modified copy?

I.e., some bit in CPUID is 1 and we have cleared it in our copy.

Close?

If so, then this should be specifically called out in the commit
message.

> +static void copy_cpu_caps(struct cpuinfo_x86 *info)

If anything, that function should be called

store_cpu_caps()

and store it into its parameter *info.

-- 
Regards/Gruss,
    Boris.

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