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

[Bruce Richardson <bruce.richardson@intel.com>]

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.
> 
> 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. 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