bpf.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
To: ast@kernel.org, daniel@iogearbox.net
Cc: bpf@vger.kernel.org, netdev@vger.kernel.org,
	bjorn.topel@intel.com, magnus.karlsson@intel.com,
	Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Subject: [RFC PATCH bpf-next 1/5] bpf, x64: use %rcx instead of %rax for tail call retpolines
Date: Thu,  2 Jul 2020 15:49:26 +0200	[thread overview]
Message-ID: <20200702134930.4717-2-maciej.fijalkowski@intel.com> (raw)
In-Reply-To: <20200702134930.4717-1-maciej.fijalkowski@intel.com>

Currently, %rax is used to store the jump target when BPF program is
emitting the retpoline instructions that are handling the indirect
tailcall.

There is a plan to use %rax for different purpose, which is storing the
tail call counter. In order to preserve this value across the tailcalls,
use %rcx instead for jump target storage in retpoline instructions.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
---
 arch/x86/include/asm/nospec-branch.h | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index e7752b4038ff..e491c3d9f227 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -314,19 +314,19 @@ static inline void mds_idle_clear_cpu_buffers(void)
  *    lfence
  *    jmp spec_trap
  *  do_rop:
- *    mov %rax,(%rsp) for x86_64
+ *    mov %rcx,(%rsp) for x86_64
  *    mov %edx,(%esp) for x86_32
  *    retq
  *
  * Without retpolines configured:
  *
- *    jmp *%rax for x86_64
+ *    jmp *%rcx for x86_64
  *    jmp *%edx for x86_32
  */
 #ifdef CONFIG_RETPOLINE
 # ifdef CONFIG_X86_64
-#  define RETPOLINE_RAX_BPF_JIT_SIZE	17
-#  define RETPOLINE_RAX_BPF_JIT()				\
+#  define RETPOLINE_RCX_BPF_JIT_SIZE	17
+#  define RETPOLINE_RCX_BPF_JIT()				\
 do {								\
 	EMIT1_off32(0xE8, 7);	 /* callq do_rop */		\
 	/* spec_trap: */					\
@@ -334,7 +334,7 @@ do {								\
 	EMIT3(0x0F, 0xAE, 0xE8); /* lfence */			\
 	EMIT2(0xEB, 0xF9);       /* jmp spec_trap */		\
 	/* do_rop: */						\
-	EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */	\
+	EMIT4(0x48, 0x89, 0x0C, 0x24); /* mov %rcx,(%rsp) */	\
 	EMIT1(0xC3);             /* retq */			\
 } while (0)
 # else /* !CONFIG_X86_64 */
@@ -352,9 +352,9 @@ do {								\
 # endif
 #else /* !CONFIG_RETPOLINE */
 # ifdef CONFIG_X86_64
-#  define RETPOLINE_RAX_BPF_JIT_SIZE	2
-#  define RETPOLINE_RAX_BPF_JIT()				\
-	EMIT2(0xFF, 0xE0);       /* jmp *%rax */
+#  define RETPOLINE_RCX_BPF_JIT_SIZE	2
+#  define RETPOLINE_RCX_BPF_JIT()				\
+	EMIT2(0xFF, 0xE1);       /* jmp *%rcx */
 # else /* !CONFIG_X86_64 */
 #  define RETPOLINE_EDX_BPF_JIT()				\
 	EMIT2(0xFF, 0xE2)        /* jmp *%edx */
-- 
2.20.1


  reply	other threads:[~2020-07-02 13:54 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-02 13:49 [RFC PATCH bpf-next 0/5] bpf: tailcalls in BPF subprograms Maciej Fijalkowski
2020-07-02 13:49 ` Maciej Fijalkowski [this message]
2020-07-02 13:49 ` [RFC PATCH bpf-next 2/5] bpf: allow for " Maciej Fijalkowski
2020-07-02 13:49 ` [RFC PATCH bpf-next 3/5] bpf: propagate poke descriptors to subprograms Maciej Fijalkowski
2020-07-02 13:49 ` [RFC PATCH bpf-next 4/5] bpf, x64: rework pro/epilogue and tailcall handling in JIT Maciej Fijalkowski
2020-07-10 23:56   ` Alexei Starovoitov
2020-07-11  3:20     ` Alexei Starovoitov
2020-07-11  3:25       ` Alexei Starovoitov
2020-07-14  1:00         ` Maciej Fijalkowski
2020-07-14  3:36           ` Alexei Starovoitov
2020-07-14 20:50             ` Maciej Fijalkowski
2020-07-14 22:34               ` Alexei Starovoitov
2020-07-02 13:49 ` [RFC PATCH bpf-next 5/5] selftests: bpf: add dummy prog for bpf2bpf with tailcall Maciej Fijalkowski
2020-07-11  0:10 ` [RFC PATCH bpf-next 0/5] bpf: tailcalls in BPF subprograms Alexei Starovoitov
2020-07-14  0:22   ` Maciej Fijalkowski

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200702134930.4717-2-maciej.fijalkowski@intel.com \
    --to=maciej.fijalkowski@intel.com \
    --cc=ast@kernel.org \
    --cc=bjorn.topel@intel.com \
    --cc=bpf@vger.kernel.org \
    --cc=daniel@iogearbox.net \
    --cc=magnus.karlsson@intel.com \
    --cc=netdev@vger.kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).