Re: [RFC PATCH] powerpc/6xx: Don't set back MSR_RI before reenabling MMU

[Christophe Leroy]

Le 01/02/2019 Ã 12:10, Michael Ellerman a ÃcritÂ:
> Christophe Leroy <christophe.leroy@xxxxxx> writes:
>
> > By delaying the setting of MSR_RI, a 1% improvment is optained on
> > null_syscall selftest on an mpc8321.
> >
> > Without this patch:
> >
> > root@vgoippro:~# ./null_syscall
> >     1134.33 ns     378.11 cycles
> >
> > With this patch:
> >
> > root@vgoippro:~# ./null_syscall
> >     1121.85 ns     373.95 cycles
> >
> > The drawback is that a machine check during that period
> > would be unrecoverable, but as only main memory is accessed
> > during that period, it shouldn't be a concern.
>
> On 64-bit server CPUs accessing main memory can cause a UE
> (Uncorrectable Error) which can trigger a machine check.
>
> So it may still be a concern, it depends how paranoid you are.
>
> > diff --git a/arch/powerpc/kernel/head_32.S b/arch/powerpc/kernel/head_32.S
> > index 146385b1c2da..ea28a6ab56ec 100644
> > --- a/arch/powerpc/kernel/head_32.S
> > +++ b/arch/powerpc/kernel/head_32.S
> > @@ -282,8 +282,6 @@ __secondary_hold_acknowledge:
> >   	stw	r1,GPR1(r11);	\
> >   	stw	r1,0(r11);	\
> >   	tovirt(r1,r11);			/* set new kernel sp */	\
> > -	li	r10,MSR_KERNEL & ~(MSR_IR|MSR_DR); /* can take exceptions */ \
> > -	MTMSRD(r10);			/* (except for mach check in rtas) */ \
> >   	stw	r0,GPR0(r11);	\
> >   	lis	r10,STACK_FRAME_REGS_MARKER@ha; /* exception frame marker */ \
> >   	addi	r10,r10,STACK_FRAME_REGS_MARKER@l; \
>
> Where does RI get enabled? I don't see it anywhere obvious.

MSR_RI is part of MSR_KERNEL, it gets then enabled when reenabling MMU 
when calling the exception handler.

#define EXC_XFER_TEMPLATE(n, hdlr, trap, copyee, tfer, ret)	\
	li	r10,trap;					\
	stw	r10,_TRAP(r11);					\
	li	r10,MSR_KERNEL;					\
	copyee(r10, r9);					\
	bl	tfer;						\
i##n:								\
	.long	hdlr;						\
	.long	ret

where tfer = transfer_to_handler.

In transfer_to_handler (kernel/entry_32.S) you have:

transfer_to_handler_cont:
3:
	mflr	r9
	lwz	r11,0(r9)		/* virtual address of handler */
	lwz	r9,4(r9)		/* where to go when done */
[...]
	mtspr	SPRN_SRR0,r11
	mtspr	SPRN_SRR1,r10
	mtlr	r9
	SYNC
	RFI				/* jump to handler, enable MMU */

So MSR_RI is restored above as r10 contains MSR_KERNEL [ | MSR_EE ]

Christophe



> cheers