linux-kernel - Re: [PATCH v3 1/8] x86/split_lock: Export handle_user_split

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Date:   Tue, 3 Mar 2020 10:42:09 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        hpa@...or.com, Paolo Bonzini <pbonzini@...hat.com>,
        Andy Lutomirski <luto@...nel.org>, tony.luck@...el.com,
        peterz@...radead.org, fenghua.yu@...el.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/8] x86/split_lock: Export handle_user_split_lock()

On Thu, Feb 06, 2020 at 03:04:05PM +0800, Xiaoyao Li wrote:
> Move the EFLAGS.AC check to do_alignment_check() so that
> handle_user_split_lock() can be used by KVM in the future to handle #AC
> caused by split lock in guest.

Probably worth explaining that KVM doesn't have a @regs context and will
pre-check EFLAGS.AC.

> Suggested-by: Sean Christopherson <sean.j.christopherson@...el.com>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> ---
>  arch/x86/include/asm/cpu.h  | 4 ++--
>  arch/x86/kernel/cpu/intel.c | 7 ++++---
>  arch/x86/kernel/traps.c     | 2 +-
>  3 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
> index ff6f3ca649b3..ff567afa6ee1 100644
> --- a/arch/x86/include/asm/cpu.h
> +++ b/arch/x86/include/asm/cpu.h
> @@ -43,11 +43,11 @@ unsigned int x86_stepping(unsigned int sig);
>  #ifdef CONFIG_CPU_SUP_INTEL
>  extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
>  extern void switch_to_sld(unsigned long tifn);
> -extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
> +extern bool handle_user_split_lock(unsigned long ip);
>  #else
>  static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
>  static inline void switch_to_sld(unsigned long tifn) {}
> -static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> +static inline bool handle_user_split_lock(unsigned long ip)
>  {
>  	return false;
>  }
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index db3e745e5d47..2b3874a96bd4 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -1058,13 +1058,13 @@ static void split_lock_init(void)
>  	sld_state = sld_off;
>  }
>  
> -bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> +bool handle_user_split_lock(unsigned long ip)
>  {
> -	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> +	if (sld_state == sld_fatal)
>  		return false;
>  
>  	pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
> -			    current->comm, current->pid, regs->ip);
> +			    current->comm, current->pid, ip);
>  
>  	/*
>  	 * Disable the split lock detection for this task so it can make
> @@ -1075,6 +1075,7 @@ bool handle_user_split_lock(struct pt_regs *regs, long error_code)
>  	set_tsk_thread_flag(current, TIF_SLD);
>  	return true;
>  }
> +EXPORT_SYMBOL_GPL(handle_user_split_lock);
>  
>  /*
>   * This function is called only when switching between tasks with
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 0ef5befaed7d..407ff9be610f 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -304,7 +304,7 @@ dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_code)
>  
>  	local_irq_enable();
>  
> -	if (handle_user_split_lock(regs, error_code))
> +	if (!(regs->flags & X86_EFLAGS_AC) && handle_user_split_lock(regs->ip))
>  		return;
>  
>  	do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
> -- 
> 2.23.0
>