Re: [gentoo-user] -march=? for this cpu

[Nilesh Govindrajan]

On Tuesday 09 April 2013 12:31:44 PM IST, Pandu Poluan wrote:
>
> On Apr 9, 2013 1:16 PM, "Michael Hampicke" <gentoo-user@××××.biz
> <mailto:gentoo-user@××××.biz>> wrote:
> >
> > Am 09.04.2013 05:50, schrieb Nilesh Govindrajan:
> > > I have two Gentoo VMs at Hetzner and the CPU supports 64 bit (grep lm
> > > /proc/cpuinfo = true).
> > >
> > > But something funny, gcc -march=native -mtune=native -v -E - 2>&1
> > > </dev/null returns this:
> > >
> > > Using built-in specs.
> > > COLLECT_GCC=/usr/x86_64-pc-linux-gnu/gcc-bin/4.6.3/gcc
> > >
> COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/4.6.3/lto-wrapper
> > > Target: x86_64-pc-linux-gnu
> > > Configured with:
> > > /var/tmp/portage/sys-devel/gcc-4.6.3/work/gcc-4.6.3/configure
> > > --prefix=/usr --bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/4.6.3
> > > --includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/include
> > > --datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.6.3
> > > --mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.6.3/man
> > > --infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.6.3/info
> > >
> --with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/include/g++-v4
> > > --host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu
> --disable-altivec
> > > --disable-fixed-point --without-ppl --without-cloog --enable-lto
> > > --enable-nls --without-included-gettext --with-system-zlib
> > > --enable-obsolete --disable-werror --enable-secureplt
> --enable-multilib
> > > --enable-libmudflap --disable-libssp --enable-libgomp
> > > --with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/4.6.3/python
> > > --enable-checking=release --disable-libgcj --enable-libstdcxx-time
> > > --enable-languages=c,c++,fortran --enable-shared
> --enable-threads=posix
> > > --enable-__cxa_atexit --enable-clocale=gnu --enable-targets=all
> > > --with-bugurl=http://bugs.gentoo.org/ --with-pkgversion='Gentoo 4.6.3
> > > p1.11, pie-0.5.2'
> > > Thread model: posix
> > > gcc version 4.6.3 (Gentoo 4.6.3 p1.11, pie-0.5.2)
> > > COLLECT_GCC_OPTIONS='-march=native' '-mtune=native' '-v' '-E'
> > >  /usr/libexec/gcc/x86_64-pc-linux-gnu/4.6.3/cc1 -E -quiet -v -
> > > -march=pentium-m -mcx16 -msahf -mno-movbe -mno-aes -mno-pclmul
> -mpopcnt
> > > -mno-abm -mno-lwp -mno-fma -mno-fma4 -mno-xop -mbmi -mno-tbm -mno-avx
> > > -mno-sse4.2 -mno-sse4.1 --param l1-cache-size=32 --param
> > > l1-cache-line-size=64 --param l2-cache-size=4096 -mtune=generic
> > > ignoring nonexistent directory "/usr/local/include"
> > > ignoring nonexistent directory
> > >
> "/usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/../../../../x86_64-pc-linux-gnu/include"
> > >
> > > #include "..." search starts here:
> > > #include <...> search starts here:
> > >  /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/include
> > >  /usr/lib/gcc/x86_64-pc-linux-gnu/4.6.3/include-fixed
> > >  /usr/include
> > > End of search list.
> > > # 1 "<stdin>"
> > > <stdin>:1:0: error: CPU you selected does not support x86-64
> instruction
> > > set
> > >
> > > It is returning pentium-m as architecture, which indeed is 32bit. I'm
> > > presently running the VM with -march=core2, but this is very weird.
> > >
> > > This is the /proc/cpuinfo:
> > >
> > > processor       : 0
> > > vendor_id       : GenuineIntel
> > > cpu family      : 6
> > > model           : 2
> > > model name      : QEMU Virtual CPU version 1.0
> > > stepping        : 3
> > > microcode       : 0x1
> > > cpu MHz         : 3399.998
> > > cache size      : 4096 KB
> > > fpu             : yes
> > > fpu_exception   : yes
> > > cpuid level     : 4
> > > wp              : yes
> > > flags           : fpu de pse tsc msr pae mce cx8 apic sep mtrr pge mca
> > > cmov pse36 clflush mmx fxsr sse sse2 syscall nx lm rep_good nopl
> pni vmx
> > > cx16 popcnt hypervisor lahf_lm
> > > bogomips        : 6799.99
> > > clflush size    : 64
> > > cache_alignment : 64
> > > address sizes   : 40 bits physical, 48 bits virtual
> > > power management:
> > >
> > > Any idea what should be the march/mtune value?
> > >
> >
> > I don't have any experience with Hetzner's VMs - I only use their
> > dedicated machines :)
> >
> > But as a more generic tip, you should install gentoo in VMs only with
> > generic optimization. You can not safely rely on what CPU your VM will
> > get. Maybe in 6 months the host your vm is on crashes, and your vm will
> > be migrated to a new system with totally different cpu (actual hardware
> > cpu or the cpu configuration that qemu is going to emulate).
> >
>
> One tip for me: If your VMs are 64-bit,  obviously the underlying CPU
> must be 64-bit. So, setting -march=nocona should be safe (Nocona is
> Intel's first AMD64-compatible CPU).
>
> Rgds,
> --
>

Well, it's a KVM virtual machine. And I'm *currently running* it with 
-march=core2 on 64bit, but that isn't very safe because it may be 
migrated any time in case the host fails. I guess it would be best to 
use generic x86_64?
Or perhaps switch to 32bit because it's just 2 GB of RAM and it can't 
be bumped up any further (top end config).

Some search reveals that it's an upstream bug where the CPU is detected 
as pentium m which happens to be 32 bit.