linux-kernel - Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt

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Date:   Wed, 1 Mar 2023 12:32:26 -0800
From:   Josh Triplett <josh@...htriplett.org>
To:     Arjan van de Ven <arjan@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Usama Arif <usama.arif@...edance.com>, dwmw2@...radead.org,
        kim.phillips@....com, brgerst@...il.com, piotrgorski@...hyos.org,
        oleksandr@...alenko.name, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
        pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v12 07/11] x86/smpboot: Remove early_gdt_descr on 64-bit

On Tue, Feb 28, 2023 at 01:02:33PM -0800, Arjan van de Ven wrote:
  Thomas Gleixner wrote:
> > 
> > Maybe we should enforce CONFIG_SMP=y first :)
> > 
> > Thanks,
> 
> for 64 bit I can see the point of removing the !SMP case entirely from arch/x86 .
> maybe even for 32 bit if it just makes the code simpler I suppose

As one of the folks keeping an eye on tinyconfig and kernel size, I
actually think we *should* make this change and rip out !CONFIG_SMP,
albeit carefully.

In particular, I would propose that we rip out !CONFIG_SMP, *but* we
allow building with CONFIG_NR_CPUS=1. (And we could make sure in that
case that the compiler can recognize that at compile time and optimize
accordingly, so that it might provide some of the UP optimizations for
us.)

Then, any *optimizations* for the "will only have one CPU, ever" case
can move to CONFIG_NR_CPUS=1 rather than !CONFIG_SMP. I think many of
those optimizations may be worth keeping for small embedded systems, or
for cases like Linux-as-bootloader or similar.

The difference here would be that code written for !CONFIG_SMP today
needs to account for the UP case for *correctness*, whereas code written
for CONFIG_SMP can *optionally* consider CONFIG_NR_CPUS=1 for
*performance*.