linux-kernel - Re: [Patch v3 Part2 4/9] x86/microcode: Do not call apply

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Date:   Wed, 1 Feb 2023 23:40:58 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Ashok Raj <ashok.raj@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Stefan Talpalaru <stefantalpalaru@...oo.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Jonathan Corbet <corbet@....net>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Peter Zilstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Martin Pohlack <mpohlack@...zon.de>
Subject: Re: [Patch v3 Part2 4/9] x86/microcode: Do not call
 apply_microcode() on sibling threads

On Wed, Feb 01, 2023 at 02:21:18PM -0800, Dave Hansen wrote:
> That works great, unless T0 experiences an error.  In that case, T0 will
> jump out of __reload_late() after failing to do the update.  T1 will
> come bumbling along after it and will enter ->apply_microcode(),
> blissfully unaware of T0's failure.  T1 will assume that it is supposed
> to do T0's job, noting "rev < mc->hdr.rev".  T1 will write the MSR while
> T0 is off doing god knows what.
> 
> T1 should not even be attempting to do ->apply_microcode() because T0 is
> not quiescent.

Yah, thanks for explaining properly.

So, if T0 fails, then we will say that it failed. The ->apply_microcode()
call on T1 was never meant to apply any microcode - just to update the
cached data.

Now, if T0 fails, then it doesn't matter what T1 does - you have a
bigger problem:

A subset of the cores is running with new microcode while other subset
with the old one. Now this is a shit situation I don't want to be in.

And I don't have a good way out of it.

Revert to the old patch? Maybe...

Retry to application on all again with the hope that it works this time?

What if some core touches a MSR being added with the new microcode
patch?

Late loading is a big PITA. As we've been preaching for a while now.

-- 
Regards/Gruss,
    Boris.

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