linux-kernel - Re: [PATCH 176/208] x86/alternatives, x86/fpu: Add 'alternatives_patched' debug flag and use it in xsave

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Date:	Wed, 6 May 2015 04:57:40 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 176/208] x86/alternatives, x86/fpu: Add
 'alternatives_patched' debug flag and use it in xsave_state()


* Borislav Petkov <bp@...en8.de> wrote:

> On Tue, May 05, 2015 at 07:58:00PM +0200, Ingo Molnar wrote:
> > We'd like to use xsave_state() earlier, but its SYSTEM_BOOTING check
> > is too imprecise.
> > 
> > The real condition that xsave_state() would like to check is whether
> > alternative XSAVE instructions were patched into the kernel image
> > already.
> > 
> > Add such a (read-mostly) debug flag and use it in xsave_state().
> > 
> > Cc: Andy Lutomirski <luto@...capital.net>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Cc: Fenghua Yu <fenghua.yu@...el.com>
> > Cc: H. Peter Anvin <hpa@...or.com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Oleg Nesterov <oleg@...hat.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > ---
> >  arch/x86/include/asm/alternative.h | 6 ++++++
> >  arch/x86/include/asm/fpu/xstate.h  | 2 +-
> >  arch/x86/kernel/alternative.c      | 5 +++++
> >  3 files changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
> > index ba32af062f61..7bfc85bbb8ff 100644
> > --- a/arch/x86/include/asm/alternative.h
> > +++ b/arch/x86/include/asm/alternative.h
> > @@ -52,6 +52,12 @@ struct alt_instr {
> >  	u8  padlen;		/* length of build-time padding */
> >  } __packed;
> >  
> > +/*
> > + * Debug flag that can be tested to see whether alternative
> > + * instructions were patched in already:
> > + */
> > +extern int alternatives_patched;
> 
> Looks useful...
> 
> > +
> >  extern void alternative_instructions(void);
> >  extern void apply_alternatives(struct alt_instr *start, struct alt_instr *end);
> >  
> > diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
> > index 31a002ad5aeb..ab2c507b58b6 100644
> > --- a/arch/x86/include/asm/fpu/xstate.h
> > +++ b/arch/x86/include/asm/fpu/xstate.h
> > @@ -119,7 +119,7 @@ static inline int xsave_state(struct xsave_struct *fx)
> >  	u32 hmask = mask >> 32;
> >  	int err = 0;
> >  
> > -	WARN_ON(system_state == SYSTEM_BOOTING);
> > +	WARN_ON(!alternatives_patched);
> 
> Btw, shouldn't we be doing this check in what is called now
> copy_kernel_to_xregs() too, i.e., right under this function which is now
> called copy_xregs_to_kernel()?

Yeah, makes sense - and we can now use WARN_ON_FPU() as well to not 
have the overhead when debugging is disabled.

Thanks,

	Ingo
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