| From | Greg Kroah-Hartman <> | Subject | [PATCH 3.18 069/150] x86, vdso: Use asm volatile in __getcpu | Date | Tue, 13 Jan 2015 23:22:20 -0800 |
| |
3.18-stable review patch. If anyone has any objections, please let me know.
------------------
From: Andy Lutomirski <luto@amacapital.net>
commit 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba upstream.
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: Marcelo Tosatti <mtosatti@redhat.com> Acked-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Andy Lutomirski <luto@amacapital.net> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
--- arch/x86/include/asm/vsyscall.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- 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;
|