Re: [PATCH] x86/smpboot: check if CLFLUSH is actually necessary

Borislav Petkov
2015-02-06T16:06:21Z

On Sat, Jan 31, 2015 at 12:31:42AM +0100, Borislav Petkov wrote:
> asm volatile(ALTERNATIVE(ASM_NOP3, "clflush %[p]", X86_BUG_CLFLUSH_MONITOR)
> : [p] "+m" (*mwait_ptr));
>
> Totally untested though - it is supposed to show the idea only.

Yeah, here's a working diff, ontop of this patchset:

https://lkml.kernel.org/r/1422987390-17878-1-git-send-email-bp@alien8.de

---
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 6d7022c683e3..771ebd6e8b77 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1432,7 +1432,11 @@ static inline void mwait_play_dead(void)
* case where we return around the loop.
*/
mb();
- clflush(mwait_ptr);
+
+ asm volatile(ALTERNATIVE("", "clflush %[p]",
+ X86_BUG_CLFLUSH_MONITOR)
+ : [p] "+m" (*(unsigned long *)mwait_ptr));
+
mb();
__monitor(mwait_ptr, 0, 0);
mb();
--

At build time you have:

--
...
ffffffff81038dcc: 65 48 8b 1c 25 88 ab mov %gs:0xab88,%rbx # movq %gs:kernel_stack,%rbx #, pfo_ret__
ffffffff81038dd3: 00 00
ffffffff81038dd5: 48 81 eb c8 3f 00 00 sub $0x3fc8,%rbx # subq $16328, %rbx #, mwait_ptr
ffffffff81038ddc: 0f 09 wbinvd
ffffffff81038dde: 45 31 e4 xor %r12d,%r12d
ffffffff81038de1: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
ffffffff81038de8: 0f ae f0 mfence
ffffffff81038deb: 90 nop
ffffffff81038dec: 90 nop
ffffffff81038ded: 90 nop
ffffffff81038dee: 0f ae f0 mfence
ffffffff81038df1: 48 89 d8 mov %rbx,%rax
...
--

which during runtime, on those affected machines only, gets patched to:

---
...
ffffffff81038dcc: mov %gs:0xab88,%rbx
ffffffff81038dd5: sub $0x3fc8,%rbx
ffffffff81038ddc: wbinvd
ffffffff81038dde: xor %r12d,%r12d
ffffffff81038de1: nopl 0x0(%rax)
ffffffff81038de8: mfence
ffffffff81038deb: clflush (%rbx) <---
ffffffff81038dee: mfence
ffffffff81038df1: mov %rbx,%rax
...
---

:-)