Re: [PATCH v3 3/4] x86/alternative: Rewrite optimize_nops() some

From: Peter Zijlstra <peterz@infradead.org>
To: Andrew.Cooper3@citrix.com
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	mhiramat@kernel.org, kirill.shutemov@linux.intel.com,
	jpoimboe@redhat.com
Subject: Re: [PATCH v3 3/4] x86/alternative: Rewrite optimize_nops() some
Date: Wed, 8 Feb 2023 22:21:12 +0100	[thread overview]
Message-ID: <Y+QSSFEfA5oXdKUS@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <Y+QPPFQ4DzSaJC3m@hirez.programming.kicks-ass.net>

On Wed, Feb 08, 2023 at 10:08:12PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 08, 2023 at 09:44:04PM +0100, Peter Zijlstra wrote:
> 
> > [   11.584069] SMP alternatives: ffffffff82000095: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> > [   11.590068] SMP alternatives: ffffffff820001f3: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> > [   11.720069] SMP alternatives: ffffffff8200189f: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> > [   11.731069] SMP alternatives: ffffffff820019ae: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> > [   11.738069] SMP alternatives: ffffffff82001a4a: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> > [   11.746069] SMP alternatives: ffffffff82001b2d: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> > [   11.766069] SMP alternatives: ffffffff82001d14: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> > [   11.770069] SMP alternatives: ffffffff82001dd5: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> > [   11.779069] SMP alternatives: ffffffff82001f35: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
> 
> UNTRAIN_RET -- specifically RESET_CALL_DEPTH

19:       48 c7 c0 80 00 00 00    mov    $0x80,%rax
20:       48 c1 e0 38             shl    $0x38,%rax
24:       65 48 89 04 25 00 00 00 00      mov    %rax,%gs:0x0     29: R_X86_64_32S        pcpu_hot+0x10

Is ofc an atrocity.

We can easily trim that by 5 bytes to:

0:   b0 80                   mov    $0x80,%al
2:   48 c1 e0 38             shl    $0x38,%rax
6:   65 48 89 04 25 00 00 00 00      mov    %rax,%gs:0x0

Who cares about the top bytes, we're explicitly shifting them out
anyway. But that's still 15 bytes or so.

If it weren't for those pesky prefix penalties that would make exactly
one instruction :-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index e04313e89f4f..be792f9407b5 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -84,7 +84,7 @@
 	movq	$-1, PER_CPU_VAR(pcpu_hot + X86_call_depth);
 
 #define RESET_CALL_DEPTH					\
-	mov	$0x80, %rax;					\
+	movb	$0x80, %al;					\
 	shl	$56, %rax;					\
 	movq	%rax, PER_CPU_VAR(pcpu_hot + X86_call_depth);
 
