lkml.org 
[lkml]   [2019]   [Feb]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC PATCH] powerpc/6xx: Don't set back MSR_RI before reenabling MMU
From
Date


Le 01/02/2019 à 12:51, Christophe Leroy a écrit :
>
>
> Le 01/02/2019 à 12:10, Michael Ellerman a écrit :
>> Christophe Leroy <christophe.leroy@c-s.fr> 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 ]
>

Looks like fast_exception_return, which is called by hash page handlers
at least, expects MSR_RI to be set. Allthough it works well on 603
(because it doesn't hash), I would most likely not work on others.

This 1% improvment is not worth it, I give up for now.

\
 
 \ /
  Last update: 2019-02-12 14:24    [W:0.068 / U:0.048 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site