linux-kernel - Re: [PATCH v7 00/17] Add support for Clang LTO

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Message-ID: <CAMj1kXHTtXqssica=ADMOrA+7mhBQv=nGBsR-XR0+LAKk_-dWA@mail.gmail.com>
Date:   Sat, 21 Nov 2020 08:35:33 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>,
        Alistair Delva <adelva@...gle.com>
Subject: Re: [PATCH v7 00/17] Add support for Clang LTO

On Sat, 21 Nov 2020 at 00:53, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> On Fri, Nov 20, 2020 at 3:30 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> >
> > On Fri, 20 Nov 2020 at 21:19, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > >
> > > On Fri, Nov 20, 2020 at 2:30 AM Ard Biesheuvel <ardb@...nel.org> wrote:
> > > >
> > > > On Thu, 19 Nov 2020 at 00:42, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > > > >
> > > > > Thanks for continuing to drive this series Sami.  For the series,
> > > > >
> > > > > Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > > > >
> > > > > I did virtualized boot tests with the series applied to aarch64
> > > > > defconfig without CONFIG_LTO, with CONFIG_LTO_CLANG, and a third time
> > > > > with CONFIG_THINLTO.  If you make changes to the series in follow ups,
> > > > > please drop my tested by tag from the modified patches and I'll help
> > > > > re-test.  Some minor feedback on the Kconfig change, but I'll post it
> > > > > off of that patch.
> > > > >
> > > >
> > > > When you say 'virtualized" do you mean QEMU on x86? Or actual
> > > > virtualization on an AArch64 KVM host?
> > >
> > > aarch64 guest on x86_64 host.  If you have additional configurations
> > > that are important to you, additional testing help would be
> > > appreciated.
> > >
> >
> > Could you run this on an actual phone? Or does Android already ship
> > with this stuff?
>
> By `this`, if you mean "the LTO series", it has been shipping on
> Android phones for years now, I think it's even required in the latest
> release.
>
> If you mean "the LTO series + mainline" on a phone, well there's the
> android-mainline of https://android.googlesource.com/kernel/common/,
> in which this series was recently removed in order to facilitate
> rebasing Android's patches on ToT-mainline until getting the series
> landed upstream.  Bit of a chicken and the egg problem there.
>
> If you mean "the LTO series + mainline + KVM" on a phone; I don't know
> the precise state of aarch64 KVM and Android (Will or Marc would
> know).  We did experiment recently with RockPI's for aach64 KVM, IIRC;
> I think Android is tricky as it still requires A64+A32/T32 chipsets,
> Alistair would know more.  Might be interesting to boot a virtualized
> (or paravirtualized?) guest built with LTO in a host built with LTO
> for sure, but I don't know if we have tried that yet (I think we did
> try LTO guests of android kernels, but I think they were on the stock
> RockPI host BSP image IIRC).
>

I don't think testing under KVM gives us more confidence or coverage
than testing on bare metal. I was just pointing out that 'virtualized'
is misleading, and if you test things under QEMU/x86 + TCG, it is
better to be clear about this, and refer to it as 'under emulation'.