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:
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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.tumm...@amd.com>
Acked-by: Anatoly Burakov <anatoly.bura...@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.
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
