> On Nov 18, 2021, at 10:58 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > On Thu, Nov 18, 2021 at 06:39:49PM +0000, Song Liu wrote: > >>> 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. > > Then you can do the exact same thing but do the writes to a temp buffer, > no different. There will be some extra work, but I guess I will give it a try. > >>>> 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)? > > You can copy in at least 4k chunks since any 4k will at most use 2 > pages, which is what it does. If that's not fast enough we can look at > doing bigger chunks. If we do JIT in a buffer first, 4kB chunks should be fast enough. Another side of this issue is the split of linear mapping (1GB => many 4kB). If we only split to PMD, but not PTE, we can probably recover most of the regression. I will check this with Johannes. Thanks, Song
