x86 fpu: command-line parameters broken post-FPU-rewrite

x86 fpu: command-line parameters broken post-FPU-rewrite

[Dave Hansen]

Hey Ingo,

We were starting to look at reenabling XSAVES support and tried to use
the 'noxsave' and 'noxsaves' kernel command-line options.  The rewrite
moved the FPU initialization to before we even are parsing command-line
options, even the early_param()s.

Do you have any preferences on how it gets fixed?

The most obvious thing would be to just defer as much of the FPU setup
as we can until after parse_early_param() has happened.  The only other
think I can think of doing would be to try to do some *really* early,
simple, command-line parsing to look for 'noxsave' and friends.

Thoughts?

Re: x86 fpu: command-line parameters broken post-FPU-rewrite

[Ingo Molnar]

* Dave Hansen <dave.hansen@intel.com> wrote:

> Hey Ingo,
> 
> We were starting to look at reenabling XSAVES support and tried to use the 
> 'noxsave' and 'noxsaves' kernel command-line options.  The rewrite moved the FPU 
> initialization to before we even are parsing command-line options, even the 
> early_param()s.
> 
> Do you have any preferences on how it gets fixed?
> 
> The most obvious thing would be to just defer as much of the FPU setup as we can 
> until after parse_early_param() has happened.  The only other think I can think 
> of doing would be to try to do some *really* early, simple, command-line parsing 
> to look for 'noxsave' and friends.
> 
> Thoughts?

Hm, so given that having the FPU operational is key to a functioning kernel (and 
other kernel subsystems may make use of FPU functionality), I'd rather bring 
option parsing earlier than change the FPU setup sequence. FPU setup should be 
done when we identify the CPU - not 'very late' as we used to.

Thanks,

	Ingo

Re: x86 fpu: command-line parameters broken post-FPU-rewrite

[Borislav Petkov]

On Wed, Dec 09, 2015 at 09:32:10AM +0100, Ingo Molnar wrote:
> 
> * Dave Hansen <dave.hansen@intel.com> wrote:
> 
> > Hey Ingo,
> > 
> > We were starting to look at reenabling XSAVES support and tried to use the 
> > 'noxsave' and 'noxsaves' kernel command-line options.  The rewrite moved the FPU 
> > initialization to before we even are parsing command-line options, even the 
> > early_param()s.
> > 
> > Do you have any preferences on how it gets fixed?
> > 
> > The most obvious thing would be to just defer as much of the FPU setup as we can 
> > until after parse_early_param() has happened.  The only other think I can think 
> > of doing would be to try to do some *really* early, simple, command-line parsing 
> > to look for 'noxsave' and friends.
> > 
> > Thoughts?
> 
> Hm, so given that having the FPU operational is key to a functioning kernel (and 
> other kernel subsystems may make use of FPU functionality), I'd rather bring 
> option parsing earlier than change the FPU setup sequence. FPU setup should be 
> done when we identify the CPU - not 'very late' as we used to.

We already have early cmdline parsing: arch/x86/lib/cmdline.c

See users for examples.

-- 
Regards/Gruss,
    Boris.

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