Re: [PATCH v7 23/33] x86/fred: let ret_from_fork() jmp to fred_exit_user when FRED is enabled

Dave Hansen
2023-04-10T18:16:21Z

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.