Creating armv7 MACHINE_ARCH

Mon Jun 12 23:11:12 UTC 2017

On Mon, Jun 12, 2017 at 5:10 PM, Russell Haley <russ.haley at gmail.com> wrote:

> Okay, feel free to ignore me, I'm not going to get the time drill into
> the source code for my own questions so I don't expect anyone else
> too. However, I'll ask anyway. I'm too confused to try and inline
> these questions. Lets see if I understand:
>
> - armv7 does not support 64 bit instructions (according to Wikipedia?
> I claim no expertise.)
> - FreeBSD has an armv6 "architecture" that is supports armv6 and armv7
> on Pre-Cortex-A-53 processors that is not supported on A-53 through
> the emulated AArch32.
> - Cortex A-53 can support armv8 (AArch64) and armv7 (AArch32) instructions
> - The current proposal is to split the armv6 and armv7 into their own
> "architectures"
>
> FreeBSD has an Arm 64 bit kernel build. I don't see what the
> TARGET_ARCH settings in the wiki and know little about it, but will
> conjecture that it doesn't have a TARGET_ARCH=armv8 (please correct me
> if I'm wrong).
>
> What I was trying to ask was: is the kernel development moving in a
> direction that clearly indicates the differences in the instructions
> vs the architectures (or have I grossly simplified the problem)? Will
> it be possible to target a Cortex-A53 and build for 32 or 64 bit
> support? Or is this just to fix RPi?
>
> In terms of Raspbian, I had assumed they were just supporting Aarch32
> across both processor models. Many of the drivers would be the same
> source if they share components so I would think it would be "simple".
> I didn't see anything in my brief look at it today to indicate
> otherwise.
>
> Thanks for letting me ask questions!
>
> Russ
>
> On Mon, Jun 12, 2017 at 2:07 PM, Warner Losh <imp at bsdimp.com> wrote:
> >
> >
> > On Mon, Jun 12, 2017 at 2:35 PM, Mark Millard <markmi at dsl-only.net>
> wrote:
> >>
> >>
> >> On 2017-Jun-12, at 1:00 PM, Mark Millard <markmi at dsl-only.net>
> wrote:
> >>
> >> > On Mon, Jun 12, 2017 at 1:00 PM, Mark Millard <markmi at dsl-only.net
> >
> >> > wrote:
> >> >> On 2017-Jun-12, at 12:16 PM, Russell Haley <russ.haley at gmail.com>
> >> >> wrote:
> >> >>
> >> >>> On Mon, Jun 12, 2017 at 10:36 AM, Mark Millard <markmi at
> >> >>> dsl-only.net> wrote:
> >> >>>>
> >> >>>> On 2017-Jun-12, at 8:39 AM, Warner Losh <imp at bsdimp.com> wrote:
> >> >>>>
> >> >>>>> . . .
> >> >>>>>
> >> >>>>> Plus, we aren't quite doing what Ian wanted. He wanted a full
> >> >>>>> rename. The
> >> >>>>> proposal on the able is to add an armv7 TARGET_ARCH in 12. Not to
> >> >>>>> rename or
> >> >>>>> remove armv6. Sadly, that will still be there since the RPI
> >> >>>>> foundation
> >> >>>>> keeps finding new ways to repackage the rpi into new boards that
> are
> >> >>>>> just
> >> >>>>> too cheap to ignore.
> >> >>>>
> >> >>>> On 2017-Jun-12, at 6:59 AM, Andrew Turner <andrew at fubar.geek.nz
> >
> >> >>>> wrote:
> >> >>>>
> >> >>>>> I like this. My understanding is adding armv7 would also fix many
> of
> >> >>>>> the currently broken ports that assume they are being built for
> armv7 as
> >> >>>>> many Linux distros target ARMv7+.
> >> >>>>>
> >> >>>>> It should also be noted the GENERIC kernel is likely to only ever
> >> >>>>> target ARMv7+ even without an armv7 TARGET_ARCH.
> >> >>>>
> >> >>>>
> >> >>>> Hopefully the choices related to TARGET and TARGET_ARCH
> >> >>>> for armv7 end up identifying the context to port builds
> >> >>>> so that many would just automatically do the right thing.
> >> >>>>
> >> >>>>
> >> >>>> As for GENERIC:
> >> >>>>
> >> >>>> powerpc has. . .
> >> >>>>
> >> >>>> TARGET=powerpc TARGET_ARCH=powerpc   and GENERIC
> >> >>>> TARGET=powerpc TARGET_ARCH=powerpc64 and GENERIC64
> >> >>>>
> >> >>>> So there is precedent for more than one GENERIC*
> >> >>>> for a family, with which one being appropriate
> >> >>>> being based on TARGET_ARCH.
> >> >>>>
> >> >>>> For powerpc TARGET=powerpc implicitly uses
> >> >>>> TARGET_ARCH=powerpc when TARGET_ARCH is not
> >> >>>> specified (if I remember right). Which should
> >> >>>> be the default for armv6 vs. armv7 might go
> >> >>>> the other direction (TARGET_ARCH=armv7 by
> >> >>>> default).
> >> >>>>
> >> >>>>
> >> >>>> Side note:
> >> >>>>
> >> >>>> A caution about talking about "rpi2" as
> >> >>>> an example. . .
> >> >>>>
> >> >>>> Raspberry Pi 2 Model B V1.2 is Cortex-A53 based
> >> >>>> (so arm64/aarch64). (A BCM2837, not a BCM2836.)
> >> >>>> This dates about to something like 2014 based
> >> >>>> on the pictures showing the (c) notice on the
> >> >>>> boards.
> >> >>>>
> >> >>>> V1.1 and before were armv7 (BCM2836) based.
> >> >>>>
> >> >>>> Unless a kernel and world are made that can
> >> >>>> also configure/handle a Cortex-A53 in a
> >> >>>> armv7-like manor there will be two different
> >> >>>> GENERIC builds in order to span the "rpi2"
> >> >>>> family, based on just V1.2+ vs. V1.1 and
> >> >>>> before.
> >> >>>>
> >> >>>> (A single, modern distribution of the official
> >> >>>> Raspbian software for the rpi2 does support
> >> >>>> all the V1.x boards if I understand right.)
> >> >>>
> >> >>> I am confused. I don't see any documentation about Raspbian
> supporting
> >> >>> 64 bit?
> >> >>
> >> >> 64-bit Cortex-A53's can be configure to operate in a
> >> >> 32-bit mode (AArch32). Raspian does that for RPI2 V1.2
> >> >> and for RPI3.
> >> >>
> >> >> Raspian does not (yet?) support a 64-bit mode (AArch64).
> >> >>
> >> >> The Cortex-A53 can support either. As I understand it
> >> >> is possible for an OS to allow a user processes to be
> >> >> one or the other, different processes using the different
> >> >> modes. That does not mean that all operating systems
> >> >> bother to.
> >> >>
> >> >> If I remember right the official Ubuntu for an ODroid-C2
> >> >> allows both AArch64 and AArch32 user programs (and
> >> >> so processes, with shared library types tracking).
> >> >>
> >> >>> From Arm at
> >> >>> https://www.arm.com/products/processors/cortex-a/cortex-
> a53-processor.php:
> >> >>> "The Cortex-A53 supports the full ARMv8-A architecture. It not only
> >> >>> runs 64-bit applications also seamlessly and efficiently runs legacy
> >> >>> ARM 32-bit applications."
> >> >>>
> >> >>> I assume that means it handles armv7-A without issue? (In fact, many
> >> >>> on this board know it does)
> >> >>
> >> >> I've not gone through the details but targeting AArch32
> >> >> probably means targeting a subset of armv7. Or may be
> >> >> to support both requires targeting a common subset of both.
> >> >> (My guess is that AArch32 is the definition of a common
> >> >> subset for 32-bit, at least for user processes.)
> >> >>
> >> >> Raspian targets just AArch32 on armv7 and Cortex-A53
> >> >> (user space). (If I've got the definition of AArch32
> >> >> right: otherwise a common subset.)
> >> >>
> >> >> FreeBSD targets armv7 and AArch64 separately (via
> >> >> separate GENERIC kernels). I'm not aware of FreeBSD
> >> >> targeting AArch32 at all on cores capable of AArch64.
> >> >> FreeBSD possibly does not restrict itself to AArch32
> >> >> (user processes) on armv7 if AArch32 is really a
> >> >> subset.
> >> >
> >> > I thought all 64 bit Arm instructions are defined in armv8?
> >>
> >> (I assume you are not trying to indicate armv8.1, armv8.2
> >> and such. Cortex-A53 is older than those and so does not
> >> have the newer things involved.)
> >>
> >> That Cortex-A53 allows armv8 64-bit arm code is not in
> >> dispute. But the operating system in involved in setting
> >> up what will actually work based on how it configures
> >> things and operates. Much of this is the kernel.
> >
> >
> > Correct.
> >
> >>
> >> Cortex-A53 also supports AArch32, i.e., 32 bit instructions.
> >> So that the 64-bit instructions all being there does not
> >> of itself prevent using a 32-bit mode instead.
> >>
> >> (The kernel likely has to deal with more specifics of
> >> processor variations than user code does not. My notes
> >> are really about the user process model, not the all
> >> the kernel details.)
> >>
> >> Raspian deals with armv7's that have no 64-bit support
> >> and with Cortex-A53's that do. It presents a user-process
> >> model that is 32-bit only, even on Cortex-A53's that allow
> >> for 64-bit (but do not required user programs to be AArch64
> >> code).
> >>
> >> Ubuntu for ODroid-C2 does not deal with armv7's but does
> >> allow both 64-bit (AArch64) and 32-bit (AArch32) user
> >> processes as I remember, on its Cortex-A53's.
> >>
> >> FreeBSD armv7 does not support Cortext-A53 or anything
> >> that allows 64-bit (that allows AArch64). This is a kernel
> >> level issue.
> >
> >
> > Not a hugely difficult issue to fix, but one nobody had fixed...
> >
> >>
> >> FreeBSD aarch64 does not support having AArch32 user
> >> processes. Nor does its kernel support processors that
> >> do not support aarch64 (so it does not support armv7).
> >
> >
> > Executing a 32-bit /bin/cat on aarch64 level support exists outside the
> > tree, according to the hallway track at BSDcan, so it will only be a
> matter
> > of time before compat32 exists there I think.
> >
> > There's a further complication is that the aarch32 unit of aarch64
> > processors is optional. Not all of them have it, so that can be a
> problem...
> > IIRC, the early aarch64 targets didn't have this feature...
> >
> >>
> >> These are simply examples of different choices about
> >> what combinations of the technical possibilities to
> >> put effort into supporting in the kernels (and
> >> possibly elsewhere). None of the alternatives is
> >> automatic. None are independent of software choices
> >> that must be made by each operating system.
> >
> >
> > Yes. They all require somebody to be interested in doing the work.
> >
> > Warner
> >
> >
> >
>