linux-kernel - Re: [RFC PATCH 03/67] x86/cpu: Move get_builtin_firmware() common code (from microcode only)

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Date:   Thu, 26 Nov 2020 00:18:12 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     isaku.yamahata@...el.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        isaku.yamahata@...il.com, Zhang Chen <chen.zhang@...el.com>,
        Kai Huang <kai.huang@...ux.intel.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [RFC PATCH 03/67] x86/cpu: Move get_builtin_firmware() common
 code (from microcode only)

On Wed, Nov 25, 2020, Borislav Petkov wrote:
> On Mon, Nov 16, 2020 at 10:25:48AM -0800, isaku.yamahata@...el.com wrote:
> > From: Zhang Chen <chen.zhang@...el.com>
> > 
> > Move get_builtin_firmware() to common.c so that it can be used to get
> > non-ucode firmware, e.g. Intel's SEAM modules, even if MICROCODE=n.
> 
> What for?
> 
> This is used for microcode built in the kernel - a non-common use case.
> Why is your thing built into the kernel and not a normal module object?

The SEAM module needs to be loaded during early boot, it can't be deferred to a
module, at least not without a lot more blood, sweat, and tears.

The SEAM Loader is an ACM that is invoked via GETSEC[EnterACCS], which requires
all APs to be in WFS.  SEAM Loader also returns control to the kernel with a
null IDT and NMIs unblocked, i.e. we're toast if there's a pending NMI.  And
unlike the run-time SEAMCALLs, boot-time SEAMCALLs do not have a strictly
bounded runtime.  Invoking configuration SEAMCALLs after the kernel is fully up
and running could cause instability as IRQ, NMI, and SMI are all blocked in SEAM
mode, e.g. a high priority IRQ/NMI/SMI could be blocked for 50+ usecs (it might
be far more than 50 usecs, I haven't seen real numbers for all SEAMCALLs).