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

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Date:	Fri, 6 Feb 2015 17:05:47 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Scotty Bauer <sbauer@....utah.edu>
Cc:	tglx@...utronix.de, mingo@...heat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] x86/smpboot: check if CLFLUSH is actually necessary

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
...
---

:-)

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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