Re: [PATCH v2 05/11] x86: add CONFIG_X86_64_NATIVE option

From: Arnd Bergmann
Date: Tue Dec 10 2024 - 15:57:38 EST

On Tue, Dec 10, 2024, at 20:05, irecca.kun@xxxxxxxxx wrote:
> Hello.
>
> On 12/10/24 14:49, Arnd Bergmann wrote:
>> As a replacement for the obsole MK8/MPSC/MCORE2 configuration options,
>> allow building a specialized kernel for the local CPU, which is useful
>> for users building their own kernels, and does not require maintaining
>> a list of possible CPU options.
>
> That potentially introduces problems. Namely compilers could apply
> auto-vectorization, which currently breaks the kernel.
> We probably need an additional patch like this:
> https://github.com/zen-kernel/zen-kernel/commit/95b7981ba2e5c86529de0e895c2d9e428aa3f7dc

I don't see how either -mno-avx2 or -fno-tree-vectorize would be
needed here: avx2 is already turned off because of -mno-avx,
and -ftree-vectorize is only enabled by default at -O3 level,
which we don't use (and which doesn't add any instructions).
There may be other flags that need to be disabled though.

With the flags we currently pass to the kernel, this is the
difference in the gcc-14 -Q --help=target output between the
x86-64 baseline and the -march=emeraldrapids, which is currently
the most featureful:

--- b 2024-12-10 21:37:27.182448452 +0100
+++ a 2024-12-10 21:41:14.118310513 +0100
@@ -12,24 +12,24 @@
-mabm [disabled]
-maccumulate-outgoing-args [disabled]
-maddress-mode= long
- -madx [disabled]
- -maes [disabled]
+ -madx [enabled]
+ -maes [enabled]
-malign-data= compat
-malign-double [disabled]
-malign-functions= 0
-malign-jumps= 0
-malign-loops= 0
-malign-stringops [enabled]
- -mamx-bf16 [disabled]
+ -mamx-bf16 [enabled]
-mamx-complex [disabled]
-mamx-fp16 [disabled]
- -mamx-int8 [disabled]
- -mamx-tile [disabled]
+ -mamx-int8 [enabled]
+ -mamx-tile [enabled]
-mandroid [disabled]
-mapx-features= none
-mapx-inline-asm-use-gpr32 [disabled]
-mapxf [disabled]
- -march= x86-64
+ -march= emeraldrapids
-masm= att
-mavx [disabled]
-mavx10.1 -mavx10.1-256
@@ -62,27 +62,27 @@
-mavxvnniint16 [disabled]
-mavxvnniint8 [disabled]
-mbionic [disabled]
- -mbmi [disabled]
- -mbmi2 [disabled]
+ -mbmi [enabled]
+ -mbmi2 [enabled]
-mbranch-cost=<0,5> 3
-mcall-ms2sysv-xlogues [disabled]
-mcet-switch [disabled]
-mcld [disabled]
- -mcldemote [disabled]
- -mclflushopt [disabled]
- -mclwb [disabled]
+ -mcldemote [enabled]
+ -mclflushopt [enabled]
+ -mclwb [enabled]
-mclzero [disabled]
-mcmodel= kernel
-mcmpccxadd [disabled]
-mcpu=
-mcrc32 [disabled]
- -mcx16 [disabled]
+ -mcx16 [enabled]
-mdaz-ftz [disabled]
-mdirect-extern-access [enabled]
-mdispatch-scheduler [disabled]
-mdump-tune-features [disabled]
- -menqcmd [disabled]
- -mevex512 [disabled]
+ -menqcmd [enabled]
+ -mevex512 [enabled]
-mf16c [disabled]
-mfancy-math-387 [disabled]
-mfentry [disabled]
@@ -94,17 +94,17 @@
-mforce-indirect-call [disabled]
-mfp-ret-in-387 [disabled]
-mfpmath= 387
- -mfsgsbase [disabled]
+ -mfsgsbase [enabled]
-mfunction-return= keep
-mfused-madd -ffp-contract=fast
-mfxsr [enabled]
-mgather -mtune-ctrl=use_gather
-mgeneral-regs-only [disabled]
- -mgfni [disabled]
+ -mgfni [enabled]
-mglibc [enabled]
-mhard-float [disabled]
-mharden-sls= none
- -mhle [disabled]
+ -mhle [enabled]
-mhreset [disabled]
-miamcu [disabled]
-mieee-fp [enabled]
@@ -123,15 +123,15 @@
-mlong-double-64 [disabled]
-mlong-double-80 [enabled]
-mlwp [disabled]
- -mlzcnt [disabled]
+ -mlzcnt [enabled]
-mmanual-endbr [disabled]
-mmemcpy-strategy=
-mmemset-strategy=
-mmitigate-rop [disabled]
-mmmx [disabled]
- -mmovbe [disabled]
- -mmovdir64b [disabled]
- -mmovdiri [disabled]
+ -mmovbe [enabled]
+ -mmovdir64b [enabled]
+ -mmovdiri [enabled]
-mmove-max= 128
-mmpx [disabled]
-mms-bitfields [disabled]
@@ -152,23 +152,23 @@
-mpc32 [disabled]
-mpc64 [disabled]
-mpc80 [disabled]
- -mpclmul [disabled]
+ -mpclmul [enabled]
-mpcommit [disabled]
- -mpconfig [disabled]
- -mpku [disabled]
- -mpopcnt [disabled]
+ -mpconfig [enabled]
+ -mpku [enabled]
+ -mpopcnt [enabled]
-mprefer-avx128 -mprefer-vector-width=128
-mprefer-vector-width= none
-mpreferred-stack-boundary= 3
-mprefetchi [disabled]
-mprefetchwt1 [disabled]
- -mprfchw [disabled]
- -mptwrite [disabled]
+ -mprfchw [enabled]
+ -mptwrite [enabled]
-mpush-args [enabled]
-mraoint [disabled]
- -mrdpid [disabled]
- -mrdrnd [disabled]
- -mrdseed [disabled]
+ -mrdpid [enabled]
+ -mrdrnd [enabled]
+ -mrdseed [enabled]
-mrecip [disabled]
-mrecip=
-mrecord-mcount [disabled]
@@ -178,11 +178,11 @@
-mrelax-cmpxchg-loop [disabled]
-mrtd [disabled]
-mrtm [disabled]
- -msahf [disabled]
+ -msahf [enabled]
-mscatter -mtune-ctrl=use_scatter
- -mserialize [disabled]
- -msgx [disabled]
- -msha [disabled]
+ -mserialize [enabled]
+ -msgx [enabled]
+ -msha [enabled]
-msha512 [disabled]
-mshstk [disabled]
-mskip-rax-setup [enabled]
@@ -212,11 +212,11 @@
-mtbm [disabled]
-mtls-dialect= gnu
-mtls-direct-seg-refs [enabled]
- -mtsxldtrk [disabled]
+ -mtsxldtrk [enabled]
-mtune-ctrl=
-mtune= generic
-muclibc [disabled]
- -muintr [disabled]
+ -muintr [enabled]
-munroll-only-small-loops [enabled]
-musermsr [disabled]
-mvaes [disabled]
@@ -224,15 +224,15 @@
-mvect8-ret-in-mem [disabled]
-mvpclmulqdq [disabled]
-mvzeroupper [enabled]
- -mwaitpkg [disabled]
- -mwbnoinvd [disabled]
+ -mwaitpkg [enabled]
+ -mwbnoinvd [enabled]
-mwidekl [disabled]
-mx32 [disabled]
-mxop [disabled]
- -mxsave [disabled]
- -mxsavec [disabled]
- -mxsaveopt [disabled]
- -mxsaves [disabled]
+ -mxsave [enabled]
+ -mxsavec [enabled]
+ -mxsaveopt [enabled]
+ -mxsaves [enabled]

I don't know what most of them do, but the ones I looked
up seem to be mainly integer operations.

Arnd