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

[Naveen N. Rao]

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