linux-kernel - Re: [PATCH] Fix undefined operation VMXOFF during reboot and crash

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Date:   Wed, 10 Jun 2020 12:36:13 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     "David P. Reed" <dpreed@...pplum.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Allison Randal <allison@...utok.net>,
        Enrico Weigelt <info@...ux.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Martin Molnar <martin.molnar.programming@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Jann Horn <jannh@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix undefined operation VMXOFF during reboot and crash

Hi David,

If you happen to make a v2 of this patch, there are a few comments
that begin with "/**" but they are not kernel-doc comments, so they
should instead begin with just "/*". Please see below.

(and you did not introduce this comment style here.)

On 6/10/20 11:12 AM, David P. Reed wrote:
> 
> Signed-off-by: David P. Reed <dpreed@...pplum.com>
> ---
>  arch/x86/include/asm/virtext.h | 24 ++++++++++++----
>  arch/x86/kernel/reboot.c       | 13 ++-------
>  arch/x86/kernel/traps.c        | 52 ++++++++++++++++++++++++++++++++--
>  3 files changed, 71 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/include/asm/virtext.h b/arch/x86/include/asm/virtext.h
> index 9aad0e0876fb..ea2d67191684 100644
> --- a/arch/x86/include/asm/virtext.h
> +++ b/arch/x86/include/asm/virtext.h
> @@ -13,12 +13,16 @@
>  #ifndef _ASM_X86_VIRTEX_H
>  #define _ASM_X86_VIRTEX_H
>  
> +#include <linux/percpu.h>
> +
>  #include <asm/processor.h>
>  
>  #include <asm/vmx.h>
>  #include <asm/svm.h>
>  #include <asm/tlbflush.h>
>  
> +DECLARE_PER_CPU_READ_MOSTLY(int, doing_emergency_vmxoff);
> +
>  /*
>   * VMX functions:
>   */
> @@ -33,8 +37,8 @@ static inline int cpu_has_vmx(void)
>  /** Disable VMX on the current CPU

just
   /* Disable VMX on the current CPU

>   *
>   * vmxoff causes a undefined-opcode exception if vmxon was not run
> - * on the CPU previously. Only call this function if you know VMX
> - * is enabled.
> + * on the CPU previously. Only call this function directly if you know VMX
> + * is enabled *and* CPU is in VMX root operation.
>   */
>  static inline void cpu_vmxoff(void)
>  {
> @@ -47,17 +51,25 @@ static inline int cpu_vmx_enabled(void)
>  	return __read_cr4() & X86_CR4_VMXE;
>  }
>  
> -/** Disable VMX if it is enabled on the current CPU
> +/** Force disable VMX if it is enabled on the current CPU.

just
   /* Force disable VMX if it is enabled on the current CPU.

> + * Note that if CPU is not in VMX root operation this
> + * VMXOFF will fault an undefined operation fault.
> + * So the 'doing_emergency_vmxoff' percpu flag is set,
> + * the trap handler for just restarts execution after
> + * the VMXOFF instruction.
>   *
> - * You shouldn't call this if cpu_has_vmx() returns 0.
> + * You shouldn't call this directly if cpu_has_vmx() returns 0.
>   */
>  static inline void __cpu_emergency_vmxoff(void)
>  {
> -	if (cpu_vmx_enabled())
> +	if (cpu_vmx_enabled()) {
> +		this_cpu_write(doing_emergency_vmxoff, 1);
>  		cpu_vmxoff();
> +		this_cpu_write(doing_emergency_vmxoff, 0);
> +	}
>  }
>  
> -/** Disable VMX if it is supported and enabled on the current CPU
> +/** Force disable VMX if it is supported and enabled on the current CPU

ditto.


>   */
>  static inline void cpu_emergency_vmxoff(void)
>  {


thanks.
-- 
~Randy