Re: [PATCH v2] arm64: Allow CONFIG_AUTOFDO_CLANG to be selected

[Will Deacon <will@kernel.org>]

(Aside: please try to avoid top-posting on the public lists as it messes up
the flow of conversation; I'll try to piece this back together.)

On Mon, Dec 09, 2024 at 09:30:50AM -0800, Rong Xu wrote:
> On Mon, Dec 9, 2024 at 8:20 AM Will Deacon <will@kernel.org> wrote:
> > On Mon, Nov 18, 2024 at 02:25:40PM -0800, Yabin Cui wrote:
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index fd9df6dcc593..c3814df5e391 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -103,6 +103,7 @@ config ARM64
> > >       select ARCH_SUPPORTS_PER_VMA_LOCK
> > >       select ARCH_SUPPORTS_HUGE_PFNMAP if TRANSPARENT_HUGEPAGE
> > >       select ARCH_SUPPORTS_RT
> > > +     select ARCH_SUPPORTS_AUTOFDO_CLANG
> > >       select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> > >       select ARCH_WANT_COMPAT_IPC_PARSE_VERSION if COMPAT
> > >       select ARCH_WANT_DEFAULT_BPF_JIT
> >
> > After this change, both arm64 and x86 select this option unconditionally
> > and with no apparent support code being added. So what is actually
> > required in order to select ARCH_SUPPORTS_AUTOFDO_CLANG and why isn't
> > it just available for all architectures instead?

> Enabling an AutoFDO build requires users to explicitly set CONFIG_AUTOFDO_CLANG.
> The support code is in Commit 315ad8780a129e82 (kbuild: Add AutoFDO
> support for Clang build).

Yes, that is precisely my point. The user has to enable
CONFIG_AUTOFDO_CLANG anyway, so what is the point in having
ARCH_SUPPORTS_AUTOFDO_CLANG. Why would an architecture _not_ want to
select that?

> We are not enabling this for all architectures because AutoFDO's optimized build
> relies on Last Branch Records (LBR) which aren't available on all architectures.

So? ETM isn't available on all arm64 machines and I doubt whether LBR is
available on _all_ x86 machines either. So there's a runtime failure
mode that needs to be handled anyway and I don't think the arch-specific
Kconfig option is really doing anything useful.

Will