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

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Date:   Fri, 19 Nov 2021 04:14:46 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Johannes Weiner <hannes@...xchg.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        bpf <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 2/7] set_memory: introduce
 set_memory_[ro|x]_noalias



> On Nov 18, 2021, at 10:58 AM, Peter Zijlstra <peterz@...radead.org> 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