linux-kernel - Re: [PATCH v3 1/1] pgo: Fix allocate

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Date:   Sat, 5 Jun 2021 16:45:51 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Jarmo Tiitto <jarmo.tiitto@...il.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Bill Wendling <wcw@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org,
        morbo@...gle.com
Subject: Re: [PATCH v3 1/1] pgo: Fix allocate_node() v2

On 2021-06-05, Jarmo Tiitto wrote:
>> Kees Cook wrote perjantaina 4. kesäkuuta 2021 21.06.37 EEST:
>> >
>> > I grep -R'd where the memory_contains() can be found and it is only
>> > found in #include <asm-generic/sections.h>
>>
>> That's true, but the way to use "asm-generic" is to include the
>> top-level "asm" file, so that architectures can override things as
>> needed.
>>
>Thanks, I didn't know that.
>
>> > I cross my fingers and await if this is my first accepted patch. :-)
>>
>> I tweaked it a bit and applied it (see the separate email).
>>
>> Thank you!
>>
>> -Kees
>>
>> --
>> Kees Cook
>>
>
>Whoa!
>Thanks, I'm glad it worked out. :-)
>
>Btw. I have almost forgotten that I once wrote code
>(that I didn't send) for the GCC gcov subsystem that also enabled
>-fprofile-generate/use for the kernel.
>However the Clang PGO looks much more approachable and
>easier to hack on since the profile data format is simpler.
>
>So starting to work on this felt just natural to me. :-)
>
>-Jarmo

Right, __llvm_prf_vnodes reserves space for static allocation.
There is no relocation referencing __llvm_prf_vnodes so there is
no straightforward way using the space for the kernel.

In userspace shared objects, the scheme works by linking
libclang_rt.profile-*.a into every shared object.  The
__start___llvm_prf_vnodes/__stop___llvm_prf_vnodes symbols are
delieberately hidden in compiler-rt InstrProfilingPlatformLinux.c.

The GCC gcov format is definitely simpler than the LLVM format:)