Creating armv7 MACHINE_ARCH

Mon Jun 12 23:10:06 UTC 2017

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
>
>
>