On 12/15/20 4:18 AM, Brendan Jackman wrote:
This adds two atomic opcodes, both of which include the BPF_FETCH
flag. XCHG without the BPF_FETCH flag would naturally encode
atomic_set. This is not supported because it would be of limited
value to userspace (it doesn't imply any barriers). CMPXCHG without
BPF_FETCH woulud be an atomic compare-and-write. We don't have such
an operation in the kernel so it isn't provided to BPF either.
There are two significant design decisions made for the CMPXCHG
instruction:
- To solve the issue that this operation fundamentally has 3
operands, but we only have two register fields. Therefore the
operand we compare against (the kernel's API calls it 'old') is
hard-coded to be R0. x86 has similar design (and A64 doesn't
have this problem).
A potential alternative might be to encode the other operand's
register number in the immediate field.
- The kernel's atomic_cmpxchg returns the old value, while the C11
userspace APIs return a boolean indicating the comparison
result. Which should BPF do? A64 returns the old value. x86 returns
the old value in the hard-coded register (and also sets a
flag). That means return-old-value is easier to JIT, so that's
what we use.
Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
Ack with a minor comment below.
Acked-by: Yonghong Song <yhs@xxxxxx>
---
arch/x86/net/bpf_jit_comp.c | 8 ++++++++
include/linux/filter.h | 2 ++
include/uapi/linux/bpf.h | 4 +++-
kernel/bpf/core.c | 20 ++++++++++++++++++++
kernel/bpf/disasm.c | 15 +++++++++++++++
kernel/bpf/verifier.c | 19 +++++++++++++++++--
tools/include/linux/filter.h | 2 ++
tools/include/uapi/linux/bpf.h | 4 +++-
8 files changed, 70 insertions(+), 4 deletions(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index eea7d8b0bb12..308241187582 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -815,6 +815,14 @@ static int emit_atomic(u8 **pprog, u8 atomic_op,
/* src_reg = atomic_fetch_add(dst_reg + off, src_reg); */
EMIT2(0x0F, 0xC1);
break;
+ case BPF_XCHG:
+ /* src_reg = atomic_xchg(dst_reg + off, src_reg); */
+ EMIT1(0x87);
+ break;
+ case BPF_CMPXCHG:
+ /* r0 = atomic_cmpxchg(dst_reg + off, r0, src_reg); */
+ EMIT2(0x0F, 0xB1);
+ break;
default:
pr_err("bpf_jit: unknown atomic opcode %02x\n", atomic_op);
return -EFAULT;
diff --git a/include/linux/filter.h b/include/linux/filter.h
index c3e87a63e0b8..16e0ba5e8937 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -265,6 +265,8 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
*
* BPF_ADD *(uint *) (dst_reg + off16) += src_reg
* BPF_ADD | BPF_FETCH src_reg = atomic_fetch_add(dst_reg + off16, src_reg);
+ * BPF_XCHG src_reg = atomic_xchg(dst_reg + off16, src_reg)
+ * BPF_CMPXCHG r0 = atomic_cmpxchg(dst_reg + off16, r0, src_reg)
*/
#define BPF_ATOMIC_OP(SIZE, OP, DST, SRC, OFF) \
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 760ae333a5ed..538b95472c8f 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -45,7 +45,9 @@
#define BPF_EXIT 0x90 /* function return */
/* atomic op type fields (stored in immediate) */
-#define BPF_FETCH 0x01 /* fetch previous value into src reg */
+#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */
+#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */
+#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */
Although the above code works fine, I would suggest to put
BPF_FETCH definition before BPF_XCHG and BPF_CMPXCHG, which
makes more sense intuitively.
/* Register numbers */
enum {
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
[...] \
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 760ae333a5ed..538b95472c8f 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -45,7 +45,9 @@
#define BPF_EXIT 0x90 /* function return */
/* atomic op type fields (stored in immediate) */
-#define BPF_FETCH 0x01 /* fetch previous value into src reg */
+#define BPF_XCHG (0xe0 | BPF_FETCH) /* atomic exchange */
+#define BPF_CMPXCHG (0xf0 | BPF_FETCH) /* atomic compare-and-write */
+#define BPF_FETCH 0x01 /* not an opcode on its own, used to build others */
same here.
/* Register numbers */
enum {
