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SubjectRe: [PATCH 176/208] x86/alternatives, x86/fpu: Add 'alternatives_patched' debug flag and use it in xsave_state()

* Borislav Petkov <bp@alien8.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@amacapital.net>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: Fenghua Yu <fenghua.yu@intel.com>
> > Cc: H. Peter Anvin <hpa@zytor.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Oleg Nesterov <oleg@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Signed-off-by: Ingo Molnar <mingo@kernel.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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