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 18:31:31 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "jiangshanlai@...il.com" <jiangshanlai@...il.com>,
        "Kang, Shan" <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.

You have kept saying not to use #ifdef if possible, and I tried to get if of them.

Somehow I ended up with overlooking this _unnecessary_ change, will remove.

Thanks!
  Xin