The skylake tune introduced on 04510bfa

Currently disables qemu-usermode by default, due to the lack
of support from QEMU to several of the Intel instruction set
extensions introduced within the few latest generations of
CPU architectures (e.g. QEMU does not support avx2)

While there is a good reason to perform the removal of qemu-usermode
from MACHINE_FEATURES, there are several components within the
build system that rely on it for proper compilation and behavior,
for example anything that uses gobject data introspection or even
the components like the chromium web browser require to run a QEMU
for the target architecture to build successfully.

There is no reason why we can't have those components built without
sacrificing the cpu (most) optimizations.

The process I followed on meta-chromebook to enable an optimized build
and whats being upstreamed by this patch is that by doing some
reverse engineering, I was able to figure out which instruction
set extensions are not compatible with QEMU Skylake-Client, by
performing a bit gcc magic from inside QEMU (target) to get the
available optimizations for the native architecture (which is
actually our target in this case).

These are all (not surprisingly) the avx2 extensions, listed as
follows:
-mno-avx
-mno-avx2
-mno-avx512f
-mno-avx512er
-mno-avx512cd
-mno-avx512pf
-mno-avx512dq
-mno-avx512bw
-mno-avx512vl
-mno-avx512ifma
-mno-avx512vbmi
-mno-avx512vbmi2
-mno-avx512vnni
-mno-avx512bitalg

Specifically disabling these manually (for now), allows us to build an
optimized system for the skylake/skylake based architectures (e.g. KabyLake)
while keeping the capability of using qemu-usermode, as a side note GCC
does show more unavailable optimization tunes, (hence why there might be
some warnings), but getting rid of these specifically seems enough to make
it run happily in qemu-usermode.

Basically the MACHINE_FEATURES variable is able to dictate how we will tune
the build for our device, if qemu-usermode is present, TUNE_CCARGS will
expand as follows:

TUNE_CCARGS=" -m64 -march=skylake  -mtune=skylake  -mno-avx -mno-avx2
-mno-avx512f -mno-avx512er -mno-avx512cd -mno-avx512pf -mno-avx512dq
-mno-avx512bw -mno-avx512vl -mno-avx512ifma -mno-avx512vbmi -mno-avx512vbmi2
-mno-avx512vnni -mno-avx512bitalg  -mfpmath=sse"

Whats this means is that the build will be tuned for skylake architectures,
enabling all possible extensions, (MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED,
ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and XSAVES) minus the ones QEMU is unable
to run, which will result in an illegal instruction error, notice the tune is
kept as skylake.

Whilst, if qemu-usermode is not found on MACHINE_FEATURES, TUNE_CCARGS will
expand to:
TUNE_CCARGS=" -m64 -march=skylake -mtune=generic -mavx2 -mfpmath=sse"

Which is exactly what its set to with the current tune, so this patch should
be harmless, and only extend current functionality, we will still keep
qemu-usermode disabled by default (hence enabling avx2) but the user would
have the capability to decide how the skylake tune will be handled.

As the GCC manual states, we should really try to avoid using mtune=generic
when possible, and this patch allows us to do so [1].

This patch also addresses a problem on which the current skylake tune includes
tune-core2.inc instead of tune-corei7.inc to get the list of AVAILTUNES
and PACKAGE_EXTRA_ARCHS.

Right now, AVAILTUNES are set as follows:
AVAILTUNES=" x86 x86-64 x86-64-x32 i586 i686 core2-32 core2-64 core2-64-x32
skylake-64"

Where the proper set should be (after this patch):
AVAILTUNES=" x86 x86-64 x86-64-x32 i586 i686 core2-32 core2-64 core2-64-x32
corei7-32 corei7-64 corei7-64-x32 skylake-64"

When (if), QEMU gains support for the AVX2 instruction set extensions these
can be easily removed to provide full support for qemu-usermode.

[1] https://gcc.gnu.org/onlinedocs/gcc-9.1.0/gcc/x86-Options.html



Signed-off-by: Alejandro Enedino Hernandez Samaniego <aehs29@gmail.com>
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>