linux-kernel - Re: [PATCH v7 23/33] x86/fred: let ret_from_fork() jmp to fred_exit

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Date:   Mon, 10 Apr 2023 11:16:15 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Xin Li <xin3.li@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, kvm@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, peterz@...radead.org,
        andrew.cooper3@...rix.com, seanjc@...gle.com, pbonzini@...hat.com,
        ravi.v.shankar@...el.com, jiangshanlai@...il.com,
        shan.kang@...el.com
Subject: Re: [PATCH v7 23/33] x86/fred: let ret_from_fork() jmp to
 fred_exit_user when FRED is enabled

On 4/4/23 03:27, Xin Li wrote:
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -299,7 +299,12 @@ SYM_CODE_START_NOALIGN(ret_from_fork)
>  	UNWIND_HINT_REGS
>  	movq	%rsp, %rdi
>  	call	syscall_exit_to_user_mode	/* returns with IRQs disabled */
> +#ifdef CONFIG_X86_FRED
> +	ALTERNATIVE "jmp swapgs_restore_regs_and_return_to_usermode", \
> +		    "jmp fred_exit_user", X86_FEATURE_FRED
> +#else
>  	jmp	swapgs_restore_regs_and_return_to_usermode
> +#endif

Does the #ifdef really buy us anything here?

I guess it might save a *TINY* amount of time at alternative processing
time.  But that doesn't really seem worth it.