Subject: Re: [PATCH 2/6] powerpc: Add enable_ppr kernel parameter to enable PPR save/restore

From: Haren Myneni <[email-masked]>
Date: Mon, 10 Sep 2012 22:42:13 -0700

On 09/09/2012 05:22 PM, Michael Neuling wrote:
> Benjamin Herrenschmidt <[email-masked]> wrote:
> 
>> On Sun, 2012-09-09 at 04:37 -0700, Haren Myneni wrote:
>>> enable_ppr kernel parameter is used to enable PPR save and restore.
>>> Supported on Power7 and later processors.
>>>
>>> By default, CPU_FTR_HAS_PPR is set for POWER7. If this parameter is not
>>> passed, disable CPU_FTR_HAS_PPR.
>>
>> What is the point ? Obscure / magic kernel command line options to turn
>> on a feature are pointless. Nobody knows about them, nobody enables
>> them.
>>
>> What you are doing is guarantee that nobody's ever going to enable your
>> code, so your whole patch series is thus irrelevant :-)
>>
>> If there's a good reason to *avoid* your option, then maybe consider
>> adding an option to *disable* the PPR save/restore, though of course
>> that would bring the argument that if it needs to be disabled maybe we
>> shouldn't do it in the first place, or you might want to think of a
>> reasonable way to intuit what the option should be.
> 
> IIRC, Haren was saying there's a 6% hit on null syscall for this.  So we
> suggested having a cmdline option to disable it for distros.
> 
> Haren, is my recollection correct?  If so, can you add this info the
> change log and change the sex of the option. 

Thanks Michael. Yes, we noticed 6% overhead with null syscall test.
Hence added cmdline option as suggested. I will add this comment in the
changelog.

Regarding the option name, I thought about various ones such as
retain_process_ppr, retain_smt_priority, save_ppr and etc. Finally added
'enable_ppr' since it enables CPU_FTR (CPU_FTR_HAS_PPR) which allows to
save/restore PPR value. Sure, I will change this option.

Thanks
Haren

> 
> Mikey
> 
>>
>> Ben.
>>
>>> Signed-off-by: Haren Myneni <[email-masked]>
>>>
>>> ---
>>>  Documentation/kernel-parameters.txt |    4 ++++
>>>  arch/powerpc/include/asm/cputable.h |    6 ++++--
>>>  arch/powerpc/kernel/setup_64.c      |   14 ++++++++++++++
>>>  3 files changed, 22 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>> index ad7e2e5..2881e5f 100644
>>> --- a/Documentation/kernel-parameters.txt
>>> +++ b/Documentation/kernel-parameters.txt
>>> @@ -809,6 +809,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>  			to discrete, to make X server driver able to add WB
>>>  			entry later. This parameter enables that.
>>>  
>>> +	enable_ppr	[PPC/PSERIES]
>>> +			Saves user defined PPR when process enters to kernel 
>>> +			and restores PPR at exit. But it impacts performance.
>>> +
>>>  	enable_timer_pin_1 [X86]
>>>  			Enable PIN 1 of APIC timer
>>>  			Can be useful to work around chipset bugs
>>> diff --git a/arch/powerpc/include/asm/cputable.h b/arch/powerpc/include/asm/cputable.h
>>> index b3c083d..880c469 100644
>>> --- a/arch/powerpc/include/asm/cputable.h
>>> +++ b/arch/powerpc/include/asm/cputable.h
>>> @@ -203,6 +203,7 @@ extern const char *powerpc_base_platform;
>>>  #define CPU_FTR_POPCNTD			LONG_ASM_CONST(0x0800000000000000)
>>>  #define CPU_FTR_ICSWX			LONG_ASM_CONST(0x1000000000000000)
>>>  #define CPU_FTR_VMX_COPY		LONG_ASM_CONST(0x2000000000000000)
>>> +#define CPU_FTR_HAS_PPR			LONG_ASM_CONST(0x4000000000000000)
>>>  
>>>  #ifndef __ASSEMBLY__
>>>  
>>> @@ -432,7 +433,8 @@ extern const char *powerpc_base_platform;
>>>  	    CPU_FTR_PURR | CPU_FTR_SPURR | CPU_FTR_REAL_LE | \
>>>  	    CPU_FTR_DSCR | CPU_FTR_SAO  | CPU_FTR_ASYM_SMT | \
>>>  	    CPU_FTR_STCX_CHECKS_ADDRESS | CPU_FTR_POPCNTB | CPU_FTR_POPCNTD | \
>>> -	    CPU_FTR_ICSWX | CPU_FTR_CFAR | CPU_FTR_HVMODE | CPU_FTR_VMX_COPY)
>>> +	    CPU_FTR_ICSWX | CPU_FTR_CFAR | CPU_FTR_HVMODE | \
>>> +	    CPU_FTR_VMX_COPY | CPU_FTR_HAS_PPR)
>>>  #define CPU_FTRS_CELL	(CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
>>>  	    CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
>>>  	    CPU_FTR_ALTIVEC_COMP | CPU_FTR_MMCRA | CPU_FTR_SMT | \
>>> @@ -454,7 +456,7 @@ extern const char *powerpc_base_platform;
>>>  	    (CPU_FTRS_POWER3 | CPU_FTRS_RS64 | CPU_FTRS_POWER4 |	\
>>>  	    CPU_FTRS_PPC970 | CPU_FTRS_POWER5 | CPU_FTRS_POWER6 |	\
>>>  	    CPU_FTRS_POWER7 | CPU_FTRS_CELL | CPU_FTRS_PA6T |		\
>>> -	    CPU_FTR_VSX)
>>> +	    CPU_FTR_VSX | CPU_FTR_HAS_PPR)
>>>  #endif
>>>  #else
>>>  enum {
>>> diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
>>> index 389bd4f..e4c1945 100644
>>> --- a/arch/powerpc/kernel/setup_64.c
>>> +++ b/arch/powerpc/kernel/setup_64.c
>>> @@ -98,6 +98,8 @@ int dcache_bsize;
>>>  int icache_bsize;
>>>  int ucache_bsize;
>>>  
>>> +static u32 enable_ppr = 0;
>>> +
>>>  #ifdef CONFIG_SMP
>>>  
>>>  static char *smt_enabled_cmdline;
>>> @@ -357,6 +359,9 @@ void __init setup_system(void)
>>>  {
>>>  	DBG(" -> setup_system()\n");
>>>  
>>> +	if (cpu_has_feature(CPU_FTR_HAS_PPR) && !enable_ppr)
>>> +		cur_cpu_spec->cpu_features &= ~CPU_FTR_HAS_PPR;
>>> +
>>>  	/* Apply the CPUs-specific and firmware specific fixups to kernel
>>>  	 * text (nop out sections not relevant to this CPU or this firmware)
>>>  	 */
>>> @@ -683,6 +688,15 @@ void __init setup_per_cpu_areas(void)
>>>  }
>>>  #endif
>>>  
>>> +/* early_ppr kernel parameter to save/restore PPR register */
>>> +static int __init early_ppr_enabled(char *str)
>>> +{
>>> +	enable_ppr = 1;
>>> +
>>> +	return 0;
>>> +}
>>> +
>>> +early_param("enable_ppr", early_ppr_enabled);
>>>  
>>>  #ifdef CONFIG_PPC_INDIRECT_IO
>>>  struct ppc_pci_io ppc_pci_io;
>>
>>
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