Re: [RFC PATCH] powerpc/6xx: Don't set back MSR_RI before reenabling MMU
flat view
From: Michael Ellerman <mpe@ellerman.id.au>
Date: 2019-02-01 11:10:38
Also in:
lkml
Christophe Leroy [off-list ref] 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.
quoted hunk ↗ jump to hunk
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. cheers