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

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Date:   Wed, 7 Dec 2022 16:05:19 -0800
From:   Ashok Raj <ashok.raj@...el.com>
To:     Borislav Petkov <bp@...en8.de>
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>, Ashok Raj <ashok.raj@...el.com>
Subject: Re: [Patch V1 4/7] x86/microcode/core: Take a snapshot before and
 after applying microcode

On Wed, Dec 07, 2022 at 09:25:54PM +0100, Borislav Petkov wrote:
> 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?

Correct.

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

Sounds good, I can certainly add it when I repost.

> 
> > +static void copy_cpu_caps(struct cpuinfo_x86 *info)
> 
> If anything, that function should be called
> 
> store_cpu_caps()
> 
The names sounds more fitting, I can change it as suggested.

Cheers,
Ashok