Re: [PATCH v5 3/3] power: amd power monitor support

[Ferruh Yigit <ferruh.yigit@amd.com>]

On 8/23/2023 5:03 PM, Tyler Retzlaff wrote:
> On Wed, Aug 23, 2023 at 10:19:39AM +0100, Ferruh Yigit wrote:
>> On 8/22/2023 11:30 PM, Konstantin Ananyev wrote:
>>> 18/08/2023 14:48, Bruce Richardson пишет:
>>>> On Fri, Aug 18, 2023 at 02:25:14PM +0100, Ferruh Yigit wrote:
>>>>> On 8/17/2023 3:18 PM, Konstantin Ananyev wrote:
>>>>>>
>>>>>>>> Caution: This message originated from an External Source. Use
>>>>>>>> proper caution
>>>>>>>> when opening attachments, clicking links, or responding.
>>>>>>>>
>>>>>>>>
>>>>>>>> On Wed, Aug 16, 2023 at 11:59:59AM -0700, Sivaprasad Tummala wrote:
>>>>>>>>> mwaitx allows EPYC processors to enter a implementation dependent
>>>>>>>>> power/performance optimized state (C1 state) for a specific
>>>>>>>>> period or
>>>>>>>>> until a store to the monitored address range.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Sivaprasad Tummala <sivaprasad.tummala@amd.com>
>>>>>>>>> Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>
>>>>>>>>> ---
>>>>>>>>>   lib/eal/x86/rte_power_intrinsics.c | 77
>>>>>>>>> +++++++++++++++++++++++++-----
>>>>>>>>>   1 file changed, 66 insertions(+), 11 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/lib/eal/x86/rte_power_intrinsics.c
>>>>>>>>> b/lib/eal/x86/rte_power_intrinsics.c
>>>>>>>>> index 6eb9e50807..b4754e17da 100644
>>>>>>>>> --- a/lib/eal/x86/rte_power_intrinsics.c
>>>>>>>>> +++ b/lib/eal/x86/rte_power_intrinsics.c
>>>>>>>>> @@ -17,6 +17,60 @@ static struct power_wait_status {
>>>>>>>>>        volatile void *monitor_addr; /**< NULL if not currently
>>>>>>>>> sleeping
>>>>>>>>> */  } __rte_cache_aligned wait_status[RTE_MAX_LCORE];
>>>>>>>>>
>>>>>>>>> +/**
>>>>>>>>> + * These functions uses UMONITOR/UMWAIT instructions and will
>>>>>>>>> enter C0.2
>>>>>>>> state.
>>>>>>>>> + * For more information about usage of these instructions, please
>>>>>>>>> +refer to
>>>>>>>>> + * Intel(R) 64 and IA-32 Architectures Software Developer's Manual.
>>>>>>>>> + */
>>>>>>>>> +static void intel_umonitor(volatile void *addr) {
>>>>>>>>> +     /* UMONITOR */
>>>>>>>>> +     asm volatile(".byte 0xf3, 0x0f, 0xae, 0xf7;"
>>>>>>>>> +                     :
>>>>>>>>> +                     : "D"(addr));
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>> +static void intel_umwait(const uint64_t timeout) {
>>>>>>>>> +     const uint32_t tsc_l = (uint32_t)timeout;
>>>>>>>>> +     const uint32_t tsc_h = (uint32_t)(timeout >> 32);
>>>>>>>>> +     /* UMWAIT */
>>>>>>>>> +     asm volatile(".byte 0xf2, 0x0f, 0xae, 0xf7;"
>>>>>>>>> +                     : /* ignore rflags */
>>>>>>>>> +                     : "D"(0), /* enter C0.2 */
>>>>>>>>> +                     "a"(tsc_l), "d"(tsc_h)); }
>>>>>>>>
>>>>>>>> question and perhaps Anatoly Burakov can chime in with expertise.
>>>>>>>>
>>>>>>>> gcc/clang have built-in intrinsics for umonitor and umwait i
>>>>>>>> believe as per our other
>>>>>>>> thread of discussion is there a benefit to also providing inline
>>>>>>>> assembly over just
>>>>>>>> using the intrinsics? I understand that the intrinsics may not
>>>>>>>> exist for the monitorx
>>>>>>>> and mwaitx below so it is probably necessary for amd.
>>>>>>>>
>>>>>>>> so the suggestion here is when they are available just use the
>>>>>>>> intrinsics.
>>>>>>>>
>>>>>>>> thanks
>>>>>>>>
>>>>>>> The gcc built-in functions
>>>>>>> __builtin_ia32_monitorx()/__builtin_ia32_mwaitx are available only
>>>>>>> when -mmwaitx
>>>>>>> is used specific for AMD platforms. On generic builds, these
>>>>>>> built-ins are not available and hence inline
>>>>>>> assembly is required here.
>>>>>>
>>>>>> Ok... but we can probably put them into a separate .c file that will
>>>>>> be compiled with that specific flag?
>>>>>> Same thing can be probably done for Intel specific instructions.
>>>>>> In general, I think it is much more preferable to use built-ins vs
>>>>>> inline assembly
>>>>>> (if possible off-course).
>>>>>>
>>>>>
>>>>> We don't compile different set of files for AMD and Intel, but there are
>>>>> runtime checks, so putting into separate file is not much different.
>>>
>>> Well, we probably don't compile .c files for particular vendor, but we
>>> definitely do compile some .c files for particular ISA extensions.
>>> Let say there are files in lib/acl that requires various '-mavx512*'
>>> flags, same for other libs and PMDs.
>>> So still not clear to me why same approach can't be applied to
>>> power_instrincts.c?
>>>
>>>>>
>>>>> It may be an option to always enable compiler flag (-mmwaitx), I think
>>>>> it won't hurt other platforms but I am not sure about implications of
>>>>> this to other platforms (what was the motivation for the compiler guys
>>>>> to enable these build-ins with specific flag?).
>>>>>
>>>>> Also this requires detecting compiler that supports 'mmwaitx' or not,
>>>>> etc..
>>>>>
>>>> This is the biggest reason why we have in the past added support for
>>>> these
>>>> instructions via asm bytes rather than intrinsics. It takes a long
>>>> time for
>>>> end-user compilers, especially those in LTS releases, to get the
>>>> necessary
>>>> intrinsics. 
>>>
>>> Yep, understand.
>>> But why then we can't have both implementations?
>>> Let say if WAITPKG is defined we can use builtins for
>>> umonitor/umwait/tpause, otherwise we fallback to inline asm implementation.
>>> Same story for MWAITX/monitorx.
>>>
>>
>> Yes this can be done,
>> it can be done either as different .c files per implementation, or as
>> #ifdef in same file.
>>
>> But eventually asm implementation is required, as fallback, and if we
>> will rely on asm implementation anyway, does it worth to have the
>> additional checks to be able to use built-in intrinsic?
>>
>> Does it helps to comment name of the built-in function to inline
>> assembly code, to document intention and another possible implementation?
> 
> the main value of preferring intrinsics is that when they are available
> they also work with msvc/windows. the msvc toolchain does not support
> inline asm. so some of the targets have to use intrinsics because that's all
> there is.
> 

How windows handles current power APIs without inline asm support, like
rte_power_intrinsics.c one?

Also will using both built-in and inline assembly work for Windows,
since there may be compiler versions that doesn't support built-in
functions, they should disable APIs altogether, and this can create a
scenario that list of exposed APIs changes based on compiler version.

>>
>>>> Consider a user running e.g. RHEL 8, who wants to take
>>>> advantages of the latest DPDK features; they should not be required to
>>>> upgrade their compiler - and possibly binutils/assembler - to do so.
>>>>
>>>> /Bruce
>>>