netdev - Re: [PATCH 14/18] maccess: allow architectures to provide kernel probing directly

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Date:   Wed, 13 May 2020 12:48:54 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christoph Hellwig <hch@....de>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-parisc@...r.kernel.org,
        linux-um <linux-um@...ts.infradead.org>,
        Netdev <netdev@...r.kernel.org>, bpf@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/18] maccess: allow architectures to provide kernel
 probing directly

On Wed, May 13, 2020 at 12:40 PM Christoph Hellwig <hch@....de> wrote:
>
> We do export something like it, currently it is called
> probe_kernel_address, and the last patch renames it to
> get_kernel_nofault.  However it is implemented as a wrapper
> around probe_kernel_address / copy_from_kernel_nofault and thus
> not quite as efficient and without the magic goto semantics.

Looking at the current users of "probe_kernel_read()", it looks like
it's almost mostly things that just want a single byte or word.

It's not 100% that: we definitely do several things that want the
"copy" semantics vs the "get" semantics: on the x86 side we have
CALL_INSN_SIZE and MAX_INSN_SIZE, and the ldttss_desc.

But the bulk of them do seem to be a single value.

I don't know if performance really matters here, but to me the whole
"most users seem to want to read a single value" is what makes me
think that maybe that should be the primary model, rather than have
the copy model be the primary one and then we implement the single
value case (badly) with a copy.

It probably doesn't matter that much. I certainly wouldn't hold this
series up over it - it can be a future thing.

         Linus