Re: [PATCH 2/4] x86: clear XD_DISABLED flag on Intel to regain NX

[Andi Kleen <andi@firstfloor.org>]

> Right, which is why in this code it validates the CPU brand and its
> family and model to make sure it's safe to read this MSR. The logic
> is identical to the code in early_init_intel() that also accesses
> MSR_IA32_MISC_ENABLE. I reviewed the CPU documentation and it seemed
> to support that MSR_IA32_MISC_ENABLE would be available under those
> conditions. That said, I only had a limited number of systems available
> to test it on. If there are adjustments to be made, we can fix them.

If you can even find out. The system will die silently.

Usually the main culprits are simulators. VMs etc. Most do ignore unknown
MSRs, but not all.

> 
> > I would rather move this code later into the early init code (before the
> > second mapping is set up, which is still in time). There the exception
> > handlers are up and you could handle a #GP if it happens.
> 
> The problem is that the page tables are set up before early_init, and Peter
> Anvin and I did not see a way to move the XD_DISABLE logic any later than
> where I've put it. Though I should let Peter speak for himself here, as I'm
> less familiar with that aspect of the code.

On x86-64 there are two kernel mappings: an early one and a final one.
The final one is only set up in C code.

On 32bit there's only a single one, but it could be changed (e.g. 
only add NX later)

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.