Re: [PATCH v3 1/1] pgo: Fix allocate_node() v2

[Fangrui Song]

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:)