On 17/01/26 4:11 pm, adubey wrote:
On 2026-01-17 15:41, Hari Bathini wrote:
On 14/01/26 5:14 pm, [email protected] wrote:
From: Abhishek Dubey <[email protected]>
In the conventional stack frame, the position of tail_call_cnt
is after the NVR save area (BPF_PPC_STACK_SAVE). Whereas, the
offset of tail_call_cnt in the trampoline frame is after the
stack alignment padding. BPF JIT logic could become complex
when dealing with frame-sensitive offset calculation of
tail_call_cnt. Having the same offset in both frames is the
desired objective.
The trampoline frame does not have a BPF_PPC_STACK_SAVE area.
Introducing it leads to under-utilization of extra memory meant
only for the offset alignment of tail_call_cnt.
Another challenge is the variable alignment padding sitting at
the bottom of the trampoline frame, which requires additional
handling to compute tail_call_cnt offset.
This patch addresses the above issues by moving tail_call_cnt
to the bottom of the stack frame at offset 0 for both types
of frames. This saves additional bytes required by BPF_PPC_STACK_SAVE
in trampoline frame, and a common offset computation for
tail_call_cnt serves both frames.
The changes in this patch are required by the third patch in the
series, where the 'reference to tail_call_info' of the main frame
is copied into the trampoline frame from the previous frame.
Signed-off-by: Abhishek Dubey <[email protected]>
---
arch/powerpc/net/bpf_jit.h | 4 ++++
arch/powerpc/net/bpf_jit_comp64.c | 31 ++++++++++++++++++++-----------
2 files changed, 24 insertions(+), 11 deletions(-)
diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
index 8334cd667bba..45d419c0ee73 100644
--- a/arch/powerpc/net/bpf_jit.h
+++ b/arch/powerpc/net/bpf_jit.h
@@ -72,6 +72,10 @@
} } while (0)
#ifdef CONFIG_PPC64
+
+/* for tailcall counter */
+#define BPF_PPC_TAILCALL 8
+
/* If dummy pass (!image), account for maximum possible
instructions */
#define PPC_LI64(d, i) do { \
if (!image) \
diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/
bpf_jit_comp64.c
index 1fe37128c876..39061cd742c1 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -20,13 +20,15 @@
#include "bpf_jit.h"
/*
- * Stack layout:
+ * Stack layout 1:
+ * Layout when setting up our own stack frame.
+ * Note: r1 at bottom, component offsets positive wrt r1.
* Ensure the top half (upto local_tmp_var) stays consistent
* with our redzone usage.
*
* [ prev sp ] <-------------
- * [ nv gpr save area ] 6*8 |
* [ tail_call_cnt ] 8 |
+ * [ nv gpr save area ] 6*8 |
* [ local_tmp_var ] 24 |
* fp (r31) --> [ ebpf stack space ] upto 512 |
* [ frame header ] 32/112 |
@@ -36,10 +38,12 @@
/* for gpr non volatile registers BPG_REG_6 to 10 */
#define BPF_PPC_STACK_SAVE (6*8)
/* for bpf JIT code internal usage */
-#define BPF_PPC_STACK_LOCALS 32
+#define BPF_PPC_STACK_LOCALS 24
/* stack frame excluding BPF stack, ensure this is quadword aligned */
#define BPF_PPC_STACKFRAME (STACK_FRAME_MIN_SIZE + \
- BPF_PPC_STACK_LOCALS + BPF_PPC_STACK_SAVE)
+ BPF_PPC_STACK_LOCALS + \
+ BPF_PPC_STACK_SAVE + \
+ BPF_PPC_TAILCALL)
/* BPF register usage */
#define TMP_REG_1 (MAX_BPF_JIT_REG + 0)
@@ -87,27 +91,32 @@ static inline bool bpf_has_stack_frame(struct
codegen_context *ctx)
}
/*
+ * Stack layout 2:
* When not setting up our own stackframe, the redzone (288 bytes)
usage is:
+ * Note: r1 from prev frame. Component offset negative wrt r1.
*
* [ prev sp ] <-------------
* [ ... ] |
* sp (r1) ---> [ stack pointer ] --------------
- * [ nv gpr save area ] 6*8
* [ tail_call_cnt ] 8
+ * [ nv gpr save area ] 6*8
* [ local_tmp_var ] 24
* [ unused red zone ] 224
*/
Calling it stack layout 1 & 2 is inappropriate. The stack layout
is essentially the same. It just goes to show things with reference
to r1 when stack is setup explicitly vs when redzone is being used...
Agree. I am using it as labels to refer in comment. Any better suggestions?
I think the comments could refer to has stack frame vs Redzone case..
- Hari
