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

["Tummala, Sivaprasad" <Sivaprasad.Tummala@amd.com>]

[AMD Official Use Only - General]

> -----Original Message-----
> From: Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>
> Sent: Thursday, August 31, 2023 4:15 AM
> To: Tyler Retzlaff <roretzla@linux.microsoft.com>; Yigit, Ferruh
> <Ferruh.Yigit@amd.com>
> Cc: Bruce Richardson <bruce.richardson@intel.com>; Konstantin Ananyev
> <konstantin.ananyev@huawei.com>; Tummala, Sivaprasad
> <Sivaprasad.Tummala@amd.com>; david.hunt@intel.com;
> anatoly.burakov@intel.com; david.marchand@redhat.com; thomas@monjalon.net;
> dev@dpdk.org
> Subject: Re: [PATCH v5 3/3] power: amd power monitor support
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> 25/08/2023 17:00, Tyler Retzlaff пишет:
> > On Thu, Aug 24, 2023 at 10:04:42AM +0100, Ferruh Yigit wrote:
> >> 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?
> >
> > so this is a windows vs toolchain entanglement.
> >
> >> 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.
> >
> > so I don't intend to disable apis, theres usually a way to make them
> > work and there should not be any api changes when done correctly.
> >
> > windows/clang/mingw
> >      * inline asm may be used, but for me isn't preferred
> >
> > windows/msvc
> >      * intrinsics (when available)
> >      * non-inline asm in a .s (when no intrinsics available)
> >      * keeping in mind that the compiler version isn't tied to windows
> >        OS release so it's easier for me to document that you need a
> >        newer compiler arbitrarily. The periods where there are no intrinsics
> >        end up being short-lived.
> >
> > I'm on the hook for windows/msvc any stickyness dealing with it ends
> > up being my problem.
>
> As I can read rte_power_instrintcts.c, for each set of power instructions we have
> related static variable: wait*_supported.
> So if we need to support compiler that supports neither new bultins nor inline-asm,
> then it probably possible to rearrange the code to keep these static vars equal zero
> for such case.
>
Konstantin,
I preferred to use the inline asm for the new power instructions with this patch due to:
1. builtins may not always available based on compiler versions
2. additional complexity with versioning checks
3. no additional performance benefit
As an improvement, will add builtins support globally in a separate patch later.

>
> >
> >>
> >>>>
> >>>>>> 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
> >>>>>