Re: [PATCH v9 06/22] x86/cea: Export __this_cpu_ist_top_va() to KVM

[Sean Christopherson]

On Fri, Mar 06, 2026, Xin Li wrote:
> > On Jan 30, 2026, at 5:46 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> diff --git a/arch/x86/include/asm/fred.h b/arch/x86/include/asm/fred.h
> index 2bb65677c079..7eea65bfc838 100644
> --- a/arch/x86/include/asm/fred.h
> +++ b/arch/x86/include/asm/fred.h
> @@ -35,6 +35,13 @@
>  
>  #ifndef __ASSEMBLER__
>  
> +enum fred_stack_level {
> +	FRED_STACK_LEVEL_0,
> +	FRED_STACK_LEVEL_1,
> +	FRED_STACK_LEVEL_2,
> +	FRED_STACK_LEVEL_3

Why bother with a layer of indirection and more enums?  Just pivot on the MSR
index.

>  #ifdef CONFIG_X86_FRED
>  #include <linux/kernel.h>
>  #include <linux/sched/task_stack.h>
> @@ -105,6 +112,8 @@ static __always_inline void fred_update_rsp0(void)
>  		__this_cpu_write(fred_rsp0, rsp0);
>  	}
>  }
> +
> +unsigned long this_cpu_fred_rsp(enum fred_stack_level lvl);
>  #else /* CONFIG_X86_FRED */
>  static __always_inline unsigned long fred_event_data(struct pt_regs *regs) { return 0; }
>  static inline void cpu_init_fred_exceptions(void) { }
> @@ -113,6 +122,7 @@ static inline void fred_complete_exception_setup(void) { }
>  static inline void fred_entry_from_kvm(unsigned int type, unsigned int vector) { }
>  static inline void fred_sync_rsp0(unsigned long rsp0) { }
>  static inline void fred_update_rsp0(void) { }
> +static unsigned long this_cpu_fred_rsp(enum fred_stack_level lvl) { return 0; }
>  #endif /* CONFIG_X86_FRED */
>  #endif /* !__ASSEMBLER__ */
>  
> diff --git a/arch/x86/kernel/fred.c b/arch/x86/kernel/fred.c
> index 433c4a6f1773..363c53701012 100644
> --- a/arch/x86/kernel/fred.c
> +++ b/arch/x86/kernel/fred.c
> @@ -72,6 +72,23 @@ void cpu_init_fred_exceptions(void)
>  	setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
>  }
>  
> +unsigned long this_cpu_fred_rsp(enum fred_stack_level lvl)
> +{
> +	switch (lvl) {
> +	case FRED_STACK_LEVEL_0:
> +		return __this_cpu_read(fred_rsp0);
> +	case FRED_STACK_LEVEL_1:
> +		return __this_cpu_ist_top_va(ESTACK_DB);
> +	case FRED_STACK_LEVEL_2:
> +		return __this_cpu_ist_top_va(ESTACK_NMI);
> +	case FRED_STACK_LEVEL_3:
> +		return __this_cpu_ist_top_va(ESTACK_DF);
> +	default:
> +		BUG();
> +	}
> +}
> +EXPORT_SYMBOL_FOR_MODULES(this_cpu_fred_rsp, "kvm-intel");

Meh, just do EXPORT_SYMBOL_FOR_KVM so that there's no export when KVM_X86=y|n.
And it's possible AMD may need to grab the MSRs too.

> +
>  /* Must be called after setup_cpu_entry_areas() */
>  void cpu_init_fred_rsps(void)
>  {
> @@ -87,7 +104,7 @@ void cpu_init_fred_rsps(void)
>  	       FRED_STKLVL(X86_TRAP_DF,  FRED_DF_STACK_LEVEL));
>  
>  	/* The FRED equivalents to IST stacks... */
> -	wrmsrq(MSR_IA32_FRED_RSP1, __this_cpu_ist_top_va(ESTACK_DB));
> -	wrmsrq(MSR_IA32_FRED_RSP2, __this_cpu_ist_top_va(ESTACK_NMI));
> -	wrmsrq(MSR_IA32_FRED_RSP3, __this_cpu_ist_top_va(ESTACK_DF));
> +	wrmsrq(MSR_IA32_FRED_RSP1, this_cpu_fred_rsp(FRED_STACK_LEVEL_1));
> +	wrmsrq(MSR_IA32_FRED_RSP2, this_cpu_fred_rsp(FRED_STACK_LEVEL_2));
> +	wrmsrq(MSR_IA32_FRED_RSP3, this_cpu_fred_rsp(FRED_STACK_LEVEL_3));
>  }