Re: [PATCH bpf-next v2] arm64, bpf: Add 12-argument support for bpf trampoline

[Alexis Lothoré]

Hello everyone,

On 7/5/24 14:53, Puranjay Mohan wrote:
> The arm64 bpf JIT currently supports attaching the trampoline to
> functions with <= 8 arguments. This is because up to 8 arguments can be
> passed in registers r0-r7. If there are more than 8 arguments then the
> 9th and later arguments are passed on the stack, with SP pointing to the
> first stacked argument. See aapcs64[1] for more details.
> 
> If the 8th argument is a structure of size > 8B, then it is passed fully
> on stack and r7 is not used for passing any argument. If there is a 9th
> argument, it will be passed on the stack, even though r7 is available.
> 
> Add the support of storing and restoring arguments passed on the stack
> to the arm64 bpf trampoline. This will allow attaching the trampoline to
> functions that take up to 12 arguments.
> 
> [1] https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#parameter-passing
> 
> Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>

[...]

+cc Xu Kuohai + cc Florent Revest, who are involved in [0]

We at Bootlin are currently working on improving eBPF selftests coverage and
aligning ARM64 support with other platforms. For this second topic, one
remaining point is the support for 12 arguments in bpf trampolines. It looks
like a big part of the work has been done and submitted through two different
versions, the first one from Xu ([0]), and this one from Puranjay ([1]). There
is still some rework needed in both versions to properly handle some alignment
constraints.

@Puranjay @Xu are you (or anyone else) actively working on those series (I kind
of understand that you both agreed that Puranjay was continuing this work) ? If
so, if there a way to assist you on this topic ? If not, can we take and revive
your work to try to handle the missing points and make this feature integrated ?

Thanks,

Alexis

[0] https://lore.kernel.org/bpf/20230917150752.69612-1-xukuohai@xxxxxxxxxxxxxxx/
[1] https://lore.kernel.org/bpf/20240705125336.46820-1-puranjay@xxxxxxxxxx/

-- 
Alexis Lothoré, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com