On Tue, Jan 13, 2015 at 02:44:09PM -0800, Andy Lutomirski wrote: > commit 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba upstream > Thank you for the backport Andy. I'll queue it for the 3.16 kernel. Cheers, -- Luís > In Linux 3.18 and below, GCC hoists the lsl instructions in the > pvclock code all the way to the beginning of __vdso_clock_gettime, > slowing the non-paravirt case significantly. For unknown reasons, > presumably related to the removal of a branch, the performance issue > is gone as of > > e76b027e6408 x86,vdso: Use LSL unconditionally for vgetcpu > > but I don't trust GCC enough to expect the problem to stay fixed. > > There should be no correctness issue, because the __getcpu calls in > __vdso_vlock_gettime were never necessary in the first place. > > Note to stable maintainers: In 3.18 and below, depending on > configuration, gcc 4.9.2 generates code like this: > > 9c3: 44 0f 03 e8 lsl %ax,%r13d > 9c7: 45 89 eb mov %r13d,%r11d > 9ca: 0f 03 d8 lsl %ax,%ebx > > This patch won't apply as is to any released kernel, but I'll send a > trivial backported version if needed. > > [ > Backported by Andy Lutomirski. Should apply to all affected > versions. This fixes a functionality bug as well as a performance > bug: buggy kernels can infinite loop in __vdso_clock_gettime on > affected compilers. See, for exammple: > > https://bugzilla.redhat.com/show_bug.cgi?id=1178975 > ] > > Fixes: 51c19b4f5927 x86: vdso: pvclock gettime support > Cc: stable@xxxxxxxxxxxxxxx # 3.8+ > Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx> > Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx> > > Conflicts: > arch/x86/include/asm/vgtod.h > --- > arch/x86/include/asm/vsyscall.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h > index 2a46ca720afc..2874be9aef0a 100644 > --- a/arch/x86/include/asm/vsyscall.h > +++ b/arch/x86/include/asm/vsyscall.h > @@ -34,7 +34,7 @@ static inline unsigned int __getcpu(void) > native_read_tscp(&p); > } else { > /* Load per CPU data from GDT */ > - asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG)); > + asm volatile ("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG)); > } > > return p; > -- > 2.1.0 > > -- > To unsubscribe from this list: send the line "unsubscribe stable" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html