linux-kernel - Re: [PATCH v3 1/5] x86/microcode/intel: Check against CPU signature before saving microcode

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Message-ID: <YwZ7u0ljd+AW1P9d@zn.tnic>
Date:   Wed, 24 Aug 2022 21:27:55 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ashok Raj <ashok.raj@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML Mailing List <linux-kernel@...r.kernel.org>,
        X86-kernel <x86@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Tom Lendacky <thomas.lendacky@....com>,
        Jacon Jun Pan <jacob.jun.pan@...el.com>
Subject: Re: [PATCH v3 1/5] x86/microcode/intel: Check against CPU signature
 before saving microcode

On Tue, Aug 23, 2022 at 11:13:13AM +0000, Ashok Raj wrote:
> > > patch1:
> > > fms3 <--- header FMS
> > > ...
> > > ext_sig:
> > > fms1
> > > fms2
> > > 
> > > patch2: new
> > > fms2 <--- header FMS
> > > 
> > > Current code takes only fms3 and checks with patch2 fms2.
> > 
> > So, find_matching_signature() does all the signatures matching and
> > scanning already. If anything, that function should tell its callers
> > whether the patch it is looking at - the fms2 one - should replace the
> > current one or not.
> > 
> > I.e., all the logic to say how strong a patch match is, should be
> > concentrated there. And then the caller will do the according action.
> 
> I updated the commit log accordingly. Basically find_matching_signature()
> is only intended to find a CPU's sig/pf against a microcode image and not
> intended to compare between two different images. 

Err, what?

find_matching_signature() looks at fmt3 - your example above - and then
goes and looks at ext_sig. Also your example above.

So you can teach that function to say with a *separate* return value
"replace current patch with this new patch because this new patch is a
better fit."

-- 
Regards/Gruss,
    Boris.

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