On 17/09/24 1:20 pm, Alexei Starovoitov wrote:

On Sun, Sep 15, 2024 at 10:58 PM Hari Bathini [off-list ref] wrote:

quoted

+
+       /*
+        * Generated stack layout:
+        *
+        * func prev back chain         [ back chain        ]
+        *                              [                   ]
+        * bpf prog redzone/tailcallcnt [ ...               ] 64 bytes (64-bit powerpc)
+        *                              [                   ] --

...

quoted

+
+       /* Dummy frame size for proper unwind - includes 64-bytes red zone for 64-bit powerpc */
+       bpf_dummy_frame_size = STACK_FRAME_MIN_SIZE + 64;

What is the goal of such a large "red zone" ?
The kernel stack is a limited resource.
Why reserve 64 bytes ?
tail call cnt can probably be optional as well.

Hi Alexei, thanks for reviewing.
FWIW, the redzone on ppc64 is 288 bytes. BPF JIT for ppc64 was using
a redzone of 80 bytes since tailcall support was introduced [1].
It came down to 64 bytes thanks to [2]. The red zone is being used
to save NVRs and tail call count when a stack is not setup. I do
agree that we should look at optimizing it further. Do you think
the optimization should go as part of PPC64 trampoline enablement
being done here or should that be taken up as a separate item, maybe?

[1] 
https://lore.kernel.org/all/40b65ab2bb3a48837ab047a70887de3ccd70c56b.1474661927.git.naveen.n.rao@linux.vnet.ibm.com/ (local)
[2] https://lore.kernel.org/all/20180503230824.3462-11-daniel@iogearbox.net/ (local)

Thanks
Hari