How should anaconda check for PAE? (was Re: arch fun.)

Gerd Hoffmann kraxel at redhat.com
Wed Feb 25 09:24:57 UTC 2009


Will Woods wrote:
> (PAE_flag and >=4GB RAM) or (PAE_flag and vmx_flag and >=1GB RAM)
> 
> where vmx_flag is the flag for hardware virt stuff. Is this a good test?

No.

> Some further questions:
> - Is a PAE kernel required for proper virt support?

Xen stopped supporting non-PAE (paravirt) guest kernels recently.  Even
before that everything ran in PAE mode because PAE was a compile time
option for Xen (i.e. mixing PAE and non-PAE guests wasn't possible).

So, yes, for Xen the PAE kernel is mandatory.  No, the vmx bit isn't a
good test, paravirtualized xen guests don't need hardware support.  For
kvm it doesn't matter I think.

I'd say we want in any case ...

 (1) Install PAE kernel if there is RAM mapped above 4G.
 (2) Install PAE kernel if the CPU supports NX (regardless
     of the installed memory).

I don't see the point in checking for vmx/svm.  Beside that I think
there is no hardware which has virtualization hardware support but
hasn't nx, so the check would be superfluous anyway.

We can also simply do this:

 - Install PAE kernel if the CPU supports PAE.

i.e. make PAE the default kernel.

cheers,
  Gerd





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