From: Peter Zijlstra <peterz@infradead.org>
To: x86@kernel.org, tony.luck@intel.com, pjt@google.com
Cc: linux-kernel@vger.kernel.org, peterz@infradead.org,
r.marek@assembler.cz, jpoimboe@redhat.com, jikos@kernel.org
Subject: [RFC][PATCH 2/2] x86/retpoline: Compress retpolines
Date: Thu, 18 Feb 2021 17:59:40 +0100 [thread overview]
Message-ID: <20210218172151.942060087@infradead.org> (raw)
In-Reply-To: <20210218165938.213678824@infradead.org>
By using int3 as a speculation fence instead of lfence, we can shrink
the longest alternative to just 15 bytes:
0: e8 05 00 00 00 callq a <.altinstr_replacement+0xa>
5: f3 90 pause
7: cc int3
8: eb fb jmp 5 <.altinstr_replacement+0x5>
a: 48 89 04 24 mov %rax,(%rsp)
e: c3 retq
This means we can change the alignment from 32 to 16 bytes and get 4
retpolines per cacheline, $I win.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/lib/retpoline.S | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -16,7 +16,7 @@
.Lspec_trap_\@:
UNWIND_HINT_EMPTY
pause
- lfence
+ int3
jmp .Lspec_trap_\@
.Ldo_rop_\@:
mov %\reg, (%_ASM_SP)
@@ -27,7 +27,7 @@
.macro THUNK reg
.section .text.__x86.indirect_thunk
- .align 32
+ .align 16
SYM_FUNC_START(__x86_indirect_thunk_\reg)
ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \
next prev parent reply other threads:[~2021-02-18 19:05 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-18 16:59 [RFC][PATCH 0/2] x86/retpoline: Retpoline on a diet Peter Zijlstra
2021-02-18 16:59 ` [RFC][PATCH 1/2] x86/retpoline: Simplify retpolines Peter Zijlstra
2021-02-22 11:36 ` Peter Zijlstra
2021-02-18 16:59 ` Peter Zijlstra [this message]
2021-02-19 7:14 ` [RFC][PATCH 2/2] x86/retpoline: Compress retpolines Borislav Petkov
2021-02-22 11:27 ` Peter Zijlstra
2021-02-18 18:46 ` [RFC PATCH] x86/retpolines: Prevent speculation after RET Borislav Petkov
2021-02-18 19:02 ` Peter Zijlstra
2021-02-18 19:11 ` Borislav Petkov
2021-02-19 8:15 ` Peter Zijlstra
2021-02-19 12:08 ` Andrew Cooper
2021-02-19 9:28 ` David Laight
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