Re: [PATCH bpf-next 2/7] set_memory: introduce set_memory_[ro|x]_noalias

On Nov 18, 2021, at 10:28 AM, Peter Zijlstra [off-list ref] wrote:

On Thu, Nov 18, 2021 at 05:16:24PM +0000, Song Liu wrote:

quoted

quoted

On Nov 17, 2021, at 11:54 PM, Peter Zijlstra [off-list ref] wrote:

On Wed, Nov 17, 2021 at 11:57:12PM +0000, Song Liu wrote:

quoted

I would agree that __text_poke() is a safer option. But in this case, we 
will need the temporary hole to be 2MB in size. Also, we will probably 
hold the temporary mapping for longer time (the whole JITing process). 
Does this sound reasonable?

No :-)

Jit to a buffer, then copy the buffer into the 2M page using 4k aliases.
IIRC each program is still smaller than a single page, right? So at no
point do you need more than 2 pages mapped anyway.

JITing to a separate buffer adds complexity to the JIT process, as we 
need to redo some offsets before the copy to match the final location of 
the program. I don't have much experience with the JIT engine, so I am
not very sure how much work it gonna be. 

You're going to have to do that anyway if you're going to write to the
directmap while executing from the alias.

Not really. If you look at current version 7/7, the logic is mostly 
straightforward. We just make all the writes to the directmap, while 
calculate offset from the alias. 

quoted

The BPF program could have up to 1000000 (BPF_COMPLEXITY_LIMIT_INSNS)
instructions (BPF instructions). So it could easily go beyond a few 
pages. Mapping the 2MB page all together should make the logic simpler. 

Then copy it in smaller chunks I suppose.

How fast/slow is the __text_poke routine? I guess we cannot do it thousands
of times per BPF program (in chunks of a few bytes)? 

Thanks,
Song