Re: [PATCH v3 14/16] x86/fred: Play nice with invoking asm_fred_entry_from_kvm() on non-FRED hardware

[Xin Li]

On 7/14/2025 3:20 AM, Peter Zijlstra wrote:
>   	call __fred_entry_from_kvm		/* Call the C entry point */
> -	POP_REGS
> -	ERETS
> -1:
> +
> +1:	/*

The symbol "1" is misplaced; it needs to be put after the ERETS
instruction.

> +	 * When FRED, use ERETS to potentially clear NMIs, otherwise simply
> +	 * restore the stack pointer.
> +	 */
> +	ALTERNATIVE "nop; nop; mov %rbp, %rsp", \

Why explicitly add two nops here?

ALTERNATIVE will still pad three-byte nop after the MOV instruction.

> +	            __stringify(add $C_PTREGS_SIZE, %rsp; ERETS), \
> +		    X86_FEATURE_FRED
> +
>   	/*
> -	 * Objtool doesn't understand what ERETS does, this hint tells it that
> -	 * yes, we'll reach here and with what stack state. A save/restore pair
> -	 * isn't strictly needed, but it's the simplest form.
> +	 * Objtool doesn't understand ERETS, and the cfi register state is
> +	 * different from initial_func_cfi due to PUSH_REGS. Tell it the state
> +	 * is similar to where UNWIND_HINT_SAVE is.
>   	 */
>   	UNWIND_HINT_RESTORE
> +
>   	pop %rbp
>   	RET
>