linux-kernel - Re: [PATCH v3 4/7] x86/boot: Set 5-level paging CPU cap before entering C code

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Message-ID: <4ul6frw5heovjirf73hygxc2ncfgd3ssb4m2nyef4catd5zasl@dqy5d7z3eknr>
Date: Thu, 15 May 2025 14:05:42 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	x86@...nel.org, Ingo Molnar <mingo@...nel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH v3 4/7] x86/boot: Set 5-level paging CPU cap before
 entering C code

On Thu, May 15, 2025 at 10:43:27AM +0100, Ard Biesheuvel wrote:
> On Thu, 15 May 2025 at 09:00, Kirill A. Shutemov <kirill@...temov.name> wrote:
> >
> > On Wed, May 14, 2025 at 12:42:47PM +0200, Ard Biesheuvel wrote:
> > > From: Ard Biesheuvel <ardb@...nel.org>
> > >
> > > In order for pgtable_l5_enabled() to be reliable wherever it is used and
> > > however early, set the associated CPU capability from asm code before
> > > entering the startup C code.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > > ---
> > >  arch/x86/include/asm/cpufeature.h | 12 +++++++++---
> > >  arch/x86/kernel/cpu/common.c      |  3 ---
> > >  arch/x86/kernel/head_64.S         | 15 +++++++++++++++
> > >  3 files changed, 24 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
> > > index 893cbca37fe9..1b5de40e7bf7 100644
> > > --- a/arch/x86/include/asm/cpufeature.h
> > > +++ b/arch/x86/include/asm/cpufeature.h
> > > @@ -2,10 +2,10 @@
> > >  #ifndef _ASM_X86_CPUFEATURE_H
> > >  #define _ASM_X86_CPUFEATURE_H
> > >
> > > +#ifdef __KERNEL__
> > > +#ifndef __ASSEMBLER__
> > >  #include <asm/processor.h>
> > >
> > > -#if defined(__KERNEL__) && !defined(__ASSEMBLER__)
> > > -
> > >  #include <asm/asm.h>
> > >  #include <linux/bitops.h>
> > >  #include <asm/alternative.h>
> > > @@ -137,5 +137,11 @@ static __always_inline bool _static_cpu_has(u16 bit)
> > >  #define CPU_FEATURE_TYPEVAL          boot_cpu_data.x86_vendor, boot_cpu_data.x86, \
> > >                                       boot_cpu_data.x86_model
> > >
> > > -#endif /* defined(__KERNEL__) && !defined(__ASSEMBLER__) */
> > > +#else /* !defined(__ASSEMBLER__) */
> > > +     .macro  setup_force_cpu_cap, cap:req
> > > +     btsl    $\cap % 32, boot_cpu_data+CPUINFO_x86_capability+4*(\cap / 32)(%rip)
> > > +     btsl    $\cap % 32, cpu_caps_set+4*(\cap / 32)(%rip)
> > > +     .endm
> > > +#endif /* !defined(__ASSEMBLER__) */
> > > +#endif /* defined(__KERNEL__) */
> > >  #endif /* _ASM_X86_CPUFEATURE_H */
> > > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> > > index f6f206743d6a..c8954dc2fb26 100644
> > > --- a/arch/x86/kernel/cpu/common.c
> > > +++ b/arch/x86/kernel/cpu/common.c
> > > @@ -1752,9 +1752,6 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
> > >       setup_clear_cpu_cap(X86_FEATURE_PCID);
> > >  #endif
> > >
> > > -     if (IS_ENABLED(CONFIG_X86_5LEVEL) && (native_read_cr4() & X86_CR4_LA57))
> > > -             setup_force_cpu_cap(X86_FEATURE_5LEVEL_PAGING);
> > > -
> > >       detect_nopl();
> > >  }
> > >
> > > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> > > index 069420853304..191d5947a762 100644
> > > --- a/arch/x86/kernel/head_64.S
> > > +++ b/arch/x86/kernel/head_64.S
> > > @@ -27,6 +27,7 @@
> > >  #include <asm/fixmap.h>
> > >  #include <asm/smp.h>
> > >  #include <asm/thread_info.h>
> > > +#include <asm/cpufeature.h>
> > >
> > >  /*
> > >   * We are not able to switch in one step to the final KERNEL ADDRESS SPACE
> > > @@ -58,6 +59,20 @@ SYM_CODE_START_NOALIGN(startup_64)
> > >        */
> > >       mov     %rsi, %r15
> > >
> > > +#ifdef CONFIG_X86_5LEVEL
> >
> > Is #ifdef really needed?
> >
> 
> 'Really needed' no but setting this capability is semantically murky
> if the kernel is not configured to support it. But you wouldn't get
> far in that case, so it doesn't matter much in practice.

If we get here with CR4.LA57=1 and CONFIG_X86_5LEVEL=n, we are screwed.
Something is very broken in stub or decompressor.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov