linux-kernel - Re: [PATCH 0/6] x86/microcode: Support for Intel Staging Feature

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Message-ID: <20250326212928.2360063-1-colinmitchell@google.com>
Date: Wed, 26 Mar 2025 14:29:28 -0700
From: Colin Mitchell <colinmitchell@...gle.com>
To: dave.hansen@...el.com
Cc: bp@...en8.de, chang.seok.bae@...el.com, colinmitchell@...gle.com, 
	dave.hansen@...ux.intel.com, linux-kernel@...r.kernel.org, mingo@...hat.com, 
	tglx@...utronix.de, x86@...nel.org
Subject: Re: [PATCH 0/6] x86/microcode: Support for Intel Staging Feature

> On 2/28/25 14:52, Borislav Petkov wrote:
> You can't load any microcode anymore if you've disabled the legacy method
> too.

Staging, if I've read the code correctly here, is only used for late loading.
There is performance reason to use staging for early kernel microcode 
loading pre-SMP. Therefore, if staging perpetually fails, it can be applied
without staging on the next reboot.

> On 2/28/25 15:23, Dave Hansen wrote:
> You seem to be saying that you'd rather be (for instance) insecure
> running old microcode than have the latency blip from a legacy microcode
> load.
> What action would you take if a staging-load fails? Retry again a few
> times? Go back to the CPU vendor and get a new image? Or just ignore it?

That's correct, but the latency tradeoff scales with the platform specific
size of the microcode patch. I'd prefer to have a more deterministic
update path and believe the potential latency blip would be significant
enough to justify the option.

Adding configuration would allow me to handle the error as needed.
A retry loop would be a first step but I could also look to migrate VMs
off the machine if the platform specific latency blip would negatively 
affect sensitive guest VMs. While an ideal solution imo would then
allow me to force legacy loading, I could also settle with it being done
through a reboot where early boot would already skip staging.