Re: [PATCH] powerpc/bpf: Only update ldimm64 during extra pass when it is an address

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Christophe Leroy wrote:


Le 24/11/2022 à 11:13, Naveen N. Rao a écrit :
Christophe Leroy wrote:
ldimm64 is not only used for loading function addresses, and

That's probably true today, but I worry that that can change upstream and we may not notice at all.

Not sure what you mean.

Today POWERPC considers that ldimm64 is _always_ loading a function address whereas upstream BPF considers that ldimm64 is a function only when it is flagged BPF_PSEUDO_FUNC.

Not sure why you think we consider ldimm64 to always be loading a function address. Perhaps it is due to the poorly chosen variable name func_addr in bpf_jit_fixup_addresses(), or due to the fact that we always update the JIT code for ldimm64. In any case, we simply overwrite imm64 load instructions to ensure we are using the updated address.


In what direction could that change in the future ?

For me if they change that it becomes an API change.

More of an extension, which is exactly what we had when BPF_PSEUDO_FUNC was introduced. Took us nearly a year before we noticed.

Because we do not do a full JIT during the extra pass today like other architectures, we are the exception - there is always the risk of bpf core changes breaking our JIT. So, I still think it is better if we do a full JIT during extra pass.


- Naveen





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