Date: Mon, 12 Jun 2017 17:11:10 -0600 From: Warner Losh <imp@bsdimp.com> To: Russell Haley <russ.haley@gmail.com> Cc: Mark Millard <markmi@dsl-only.net>, "freebsd-arm@freebsd.org" <freebsd-arm@freebsd.org> Subject: Re: Creating armv7 MACHINE_ARCH Message-ID: <CANCZdfroZYX70cW0HC7qmH70yh5Y%2BAKgCB2tv_xDGYaN47R4mQ@mail.gmail.com> In-Reply-To: <CABx9NuRJLgc0cSsykrSbJi=fVq-jfOi7ZEiZ5LRHG5tqibZ_2Q@mail.gmail.com> References: <CANCZdfpUjPBRpxpmjtwK-wpiK=%2BwHscS4UmVeatrE7vrm260tw@mail.gmail.com> <20170612152808.6094931.74364.27128@gmail.com> <CANCZdfrxTo8vLsnjU_VerO%2B3%2BU=06cok7%2BuKba3FM8_nXFozhQ@mail.gmail.com> <B19EDB95-2A23-4F8F-8414-3F4E0E65AC4B@dsl-only.net> <CABx9NuQTOkf6HK=RacUCBR=W_WDfgZwbVHYwsRdx0YJd=zr51w@mail.gmail.com> <2A90A527-7DCA-4442-9322-0EA96236583C@dsl-only.net> <3CC8DE8A-CCF2-4856-A43E-6B259BDE8B2C@dsl-only.net> <CANCZdfp6cnXdHxSQGTXHq4Md4Jh6=u4Af_rDM3k_RgoN%2BFrXWA@mail.gmail.com> <CABx9NuRJLgc0cSsykrSbJi=fVq-jfOi7ZEiZ5LRHG5tqibZ_2Q@mail.gmail.com>
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On Mon, Jun 12, 2017 at 5:10 PM, Russell Haley <russ.haley@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@bsdimp.com> wrote: > > > > > > On Mon, Jun 12, 2017 at 2:35 PM, Mark Millard <markmi@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 > > > > > > >
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