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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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