Re: [PATCH 3.19.y-ckt 130/130] Revert "sched/x86_64: Don't save flags on context switch"

[Kamal Mostafa <kamal@canonical.com>]

On Sat, 2015-08-29 at 09:20 -0700, Andy Lutomirski wrote:
> If you haven't pushed this out yet, I would hold off.  I think it may
> be unnecessary.

Okay, I've dropped this revert from the 3.19-stable queue.  And for
reference, your post: http://seclists.org/oss-sec/2015/q3/456

Thanks very much for the heads-up, Andy.

 -Kamal


> On Thu, Aug 27, 2015 at 3:12 PM, Kamal Mostafa <kamal@canonical.com> wrote:
> > 3.19.8-ckt6 -stable review patch.  If anyone has any objections, please let me know.
> >
> > ------------------
> >
> > From: Andy Lutomirski <luto@kernel.org>
> >
> > commit 512255a2ad2c832ca7d4de9f31245f73781922d0 upstream.
> >
> > This reverts commit:
> >
> >   2c7577a75837 ("sched/x86_64: Don't save flags on context switch")
> >
> > It was a nice speedup.  It's also not quite correct: SYSENTER
> > enables interrupts too early.
> >
> > We can re-add this optimization once the SYSENTER code is beaten
> > into shape, which should happen in 4.3 or 4.4.
> >
> > Signed-off-by: Andy Lutomirski <luto@kernel.org>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Link: http://lkml.kernel.org/r/85f56651f59f76624e80785a8fd3bdfdd089a818.1439838962.git.luto@kernel.org
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > Signed-off-by: Kamal Mostafa <kamal@canonical.com>
> > ---
> >  arch/x86/include/asm/switch_to.h | 12 ++++--------
> >  1 file changed, 4 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> > index 751bf4b..d7f3b3b 100644
> > --- a/arch/x86/include/asm/switch_to.h
> > +++ b/arch/x86/include/asm/switch_to.h
> > @@ -79,12 +79,12 @@ do {                                                                        \
> >  #else /* CONFIG_X86_32 */
> >
> >  /* frame pointer must be last for get_wchan */
> > -#define SAVE_CONTEXT    "pushq %%rbp ; movq %%rsi,%%rbp\n\t"
> > -#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp\t"
> > +#define SAVE_CONTEXT    "pushf ; pushq %%rbp ; movq %%rsi,%%rbp\n\t"
> > +#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp ; popf\t"
> >
> >  #define __EXTRA_CLOBBER  \
> >         , "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \
> > -         "r12", "r13", "r14", "r15", "flags"
> > +         "r12", "r13", "r14", "r15"
> >
> >  #ifdef CONFIG_CC_STACKPROTECTOR
> >  #define __switch_canary                                                          \
> > @@ -100,11 +100,7 @@ do {                                                                       \
> >  #define __switch_canary_iparam
> >  #endif /* CC_STACKPROTECTOR */
> >
> > -/*
> > - * There is no need to save or restore flags, because flags are always
> > - * clean in kernel mode, with the possible exception of IOPL.  Kernel IOPL
> > - * has no effect.
> > - */
> > +/* Save restore flags to clear handle leaking NT */
> >  #define switch_to(prev, next, last) \
> >         asm volatile(SAVE_CONTEXT                                         \
> >              "movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */       \
> > --
> > 1.9.1
> >
> 
> 
>