Re: [gentoo-user] Changing CPU and motherboard

[whiteman808 <whiteman808@paraboletancza.org>]

On 28 Jun 2025, 14:32:57, Dale wrote:
> Jay Faulkner wrote:
> 
>     On 6/28/2025 9:38 AM, whiteman808 wrote:
> 
>         Is it necessary to reinstall Gentoo if I change CPU or motherboard? If
>         not, what steps should be done on the existing Gentoo installation? Do I
>         need to do these operations from chroot?
> 
>     I'm assuming this is amd64->amd64.
> 
>     So the main thing to worry about is CPU compatibility, and your CFLAGS. If
>     you're using -march=native, there's a chance your system won't work as
>     compiled. This isn't always true, but these days it's no longer a guarantee
>     that a newer CPU will have all the features of the old.
> 
> 
>     What I usually do in this case is:
> 
>     - set -march=x86-64-v3 (or whatever lowest-common-denominator CPU arch
>     generic target works) -- https://wiki.gentoo.org/wiki/Safe_CFLAGS#
>     Generic_psABI_levels can help with this. - Ensure my system is fully
>     updated, and `emerge --depclean`'d. - emerge -e @world # this will rebuild
>     your entire system.
> 
> 
>     You can *significantly* reduce the pain of this by using the binary package
>     host.
> 
> 
>     -JayF
> 
> 
> 
> Follow Jay's advice on the march setting in make.conf.  You want a setting that
> is compatible with both CPUs as Jay mentioned.  Do a emerge -e world.  Also,
> --depclean as well. 
> 
> When you install the new CPU.  You will need a kernel.  Your current one may
> work, may not.  If it does, I'd stick to a console with no GUI running.  Method
> below may be faster since you only have to recompile once if you want to switch
> the march setting back to native.
> 
> A somewhat shorter method, after you install new CPU, boot another boot media,
> Gentoo Live CD, Knoppix or whatever your system can boot with and you like. 
> Then mount your partitions as needed for the OS, chroot in, emerge -e world. 
> It should see the new CPU and change all the settings.  Since the boot media is
> handling the boot, it doesn't matter what your version of OS is as long as the
> Arch is the same.  Don't forget to build a new kernel as well.  The command
> lspci -k can be a really awesome friend.  Also, you can leave the MARCH set to
> native setting and this way you only compile once.
> 
> If you can make it without the computer a bit, I'd boot from other media and
> rebuild only once.  Oh, don't forget to change the CPU flags if you have it
> set.  The command cpuid2cpuflags is good for that.  If you don't have it,
> package is the same name.  You can unmerge it when done if you want. 
> 
> Just another idea.  There are likely several ways to accomplish this.  Biggest
> thing is the kernel. 
> 
> Dale
> 
> :-)  :-) 
Does portage need to be able to execute some files already present on
the system if I compile new software or recompile entire system using
emerge -e? If yes, what if old CPU features and flags are entirely
incompatible and not subset of the new CPU settings if I compile using
livecd? How portage handles these cases?