Re: [RFC PATCH 4/4] x86/entry/nmi: unmask NMIs on userspace NMI when entry debugging

[Andy Lutomirski <luto@kernel.org>]

On 5/31/21 11:52 PM, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>

Why?

> 
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
>  arch/x86/entry/entry_64.S | 36 ++++++++++++++++++++----------------
>  1 file changed, 20 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index f54e06139d4b..309e63f4f391 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1055,6 +1055,24 @@ SYM_CODE_START_LOCAL(error_return)
>  	jmp	swapgs_restore_regs_and_return_to_usermode
>  SYM_CODE_END(error_return)
>  
> +.macro debug_entry_unmask_NMIs
> +#ifdef CONFIG_DEBUG_ENTRY
> +	/*
> +	 * For ease of testing, unmask NMIs right away.  Disabled by
> +	 * default because IRET is very expensive.
> +	 */
> +	pushq	$0		/* SS */
> +	pushq	%rsp		/* RSP (minus 8 because of the previous push) */
> +	addq	$8, (%rsp)	/* Fix up RSP */
> +	pushfq			/* RFLAGS */
> +	pushq	$__KERNEL_CS	/* CS */
> +	pushq	$1f		/* RIP */
> +	iretq			/* continues with NMI unmasked */
> +	UNWIND_HINT_IRET_REGS
> +1:
> +#endif
> +.endm
> +
>  /*
>   * Runs on exception stack.  Xen PV does not go through this path at all,
>   * so we can use real assembly here.
> @@ -1145,6 +1163,7 @@ SYM_CODE_START(asm_exc_nmi)
>  	 * At this point we no longer need to worry about stack damage
>  	 * due to nesting -- we're done with the NMI stack.
>  	 */
> +	debug_entry_unmask_NMIs
>  	pushq	$-1		/* pt_regs->orig_ax */
>  	idtentry_body exc_nmi has_error_code=0
>  
> @@ -1286,22 +1305,7 @@ first_nmi:
>  	UNWIND_HINT_IRET_REGS
>  
>  	/* Everything up to here is safe from nested NMIs */
> -
> -#ifdef CONFIG_DEBUG_ENTRY
> -	/*
> -	 * For ease of testing, unmask NMIs right away.  Disabled by
> -	 * default because IRET is very expensive.
> -	 */
> -	pushq	$0		/* SS */
> -	pushq	%rsp		/* RSP (minus 8 because of the previous push) */
> -	addq	$8, (%rsp)	/* Fix up RSP */
> -	pushfq			/* RFLAGS */
> -	pushq	$__KERNEL_CS	/* CS */
> -	pushq	$1f		/* RIP */
> -	iretq			/* continues at repeat_nmi below */
> -	UNWIND_HINT_IRET_REGS
> -1:
> -#endif
> +	debug_entry_unmask_NMIs
>  
>  repeat_nmi:
>  	/*
>