linux-kernel - Re: [PATCH 1/3] x86/cpu: Process all CPUID dependencies after identifying CPU info

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Message-ID: <Y7YJRwlWVqt3uY9/@google.com>
Date:   Wed, 4 Jan 2023 23:18:31 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 1/3] x86/cpu: Process all CPUID dependencies after
 identifying CPU info

On Wed, Jan 04, 2023, Borislav Petkov wrote:
> On Wed, Jan 04, 2023 at 09:02:04PM +0000, Sean Christopherson wrote:
> > And there's a non-zero chance we'd end up with a kernel param to allow booting
> > unknown CPUs, e.g. for people doing weird things with VMs or running old, esoteric
> > hardware.  At that point we'd end up with a more complex implementation than
> > processing dependencies on synthetic flags, especially if there's ever a more
> > legitimate need to process such dependencies.
> 
> I'm sorry but I'm still unclear on what actual use care are we even fixing here?

There's no fix.  What I was trying to say is that modifying the kernel to refuse
to boot on unknown CPUs is opening a can of worms for very little benefit.

> If it is about people who'd like to tinker with old hw or doing weird VM things,
> they can just as well adjust their kernel .configs and rebuild.
> 
> Peeking around your patchset, if all this is about dropping the
> X86_FEATURE_MSR_IA32_FEAT_CTL check and checking only X86_FEATURE_VMX and in
> order to do that, you want to cover those obscure cases where
> init_ia32_feat_ctl() won't get run, then sure, I guess - changes look simple
> enough. :)

Yes, this is purely to drop the explicit X86_FEATURE_MSR_IA32_FEAT_CTL checks.

Alternatively, we could just drop the checks without processing the dependency,
i.e. take the stance that running KVM with a funky .config is a user error, but
that feels unnecessarily hostile since it's quite easy to play nice.

Or I guess do nothing and carry the explicit checks.