Date: Thu, 12 Feb 2015 16:12:50 -0800 From: Nathan Whitehorn <nwhitehorn@freebsd.org> To: Warner Losh <imp@bsdimp.com>, Andrew Turner <andrew@fubar.geek.nz> Cc: freebsd-arm@freebsd.org Subject: Re: FreeBSD/arm64 MACHINE/MACHINE_ARCH identification Message-ID: <54DD4182.7040505@freebsd.org> In-Reply-To: <51DBDD85-A1FD-4A18-946D-FB78252BB845@bsdimp.com> References: <CAPyFy2A=Ev5gdYPKgEE0LS3-1sY%2BXmkZA7VCe71E6Fmbb=vMRw@mail.gmail.com> <607BF592-A09B-4DB4-9872-C9E63066AB57@bsdimp.com> <CAPyFy2Bgrap3TkFNuChyMC0Vwbjdt5FVW0ey03XtkK1iwNL1KQ@mail.gmail.com> <71E9C1B9-F819-420B-90A5-A36D58E71817@bsdimp.com> <CAPyFy2ATn5xgsvePCdvzqnyBS45izVHdL8yLaQQoKeJenSv9tg@mail.gmail.com> <228428CC-4042-4902-90A4-E7040F4BFFF5@bsdimp.com> <CAPyFy2BKzhiA4tbi-mXd6T114_zawmWTi3XbyXiUcgijQfHdyw@mail.gmail.com> <54DCE9B5.8040203@freebsd.org> <EC5AAE72-F553-4F31-8768-9854B6EE2C69@bsdimp.com> <20150212225655.26c865aa@bender.lan> <51DBDD85-A1FD-4A18-946D-FB78252BB845@bsdimp.com>
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On 02/12/15 15:11, Warner Losh wrote: > >> On Feb 12, 2015, at 3:56 PM, Andrew Turner <andrew@fubar.geek.nz> wrote: >> >> On Thu, 12 Feb 2015 11:37:38 -0700 >> Warner Losh <imp@bsdimp.com> wrote: >> >>> >>>> On Feb 12, 2015, at 10:58 AM, Nathan Whitehorn >>>> <nwhitehorn@freebsd.org> wrote: >>>> >>>> On 02/12/15 09:15, Ed Maste wrote: >>>>>>> Oh - I don't care what directory Linux puts the kernel source >>>>>>> in, only what's reported by uname. As far as I can tell that >>>>>>> has always been aarch64 for uname -m. >>>>>> >>>>>> Traditionally in Linux, they have been a matched set. >>>>> >>>>> Ok, it appears they may have abandoned this. >>>>> >>>>>>> We might decide that "uname -m" has to be aarch64 to match >>>>>>> expectations of third-party software set by other operating >>>>>>> systems. If that in turn means we have to move the kernel >>>>>>> source, so be it. >>>>>> >>>>>> This one I’m not on board with. You’ve not made a compelling case >>>>>> for it yet. >>>>> >>>>> That's why I said "we might decide" -- I'm not sure myself. >>>>> >>>>> However, there's no backwards compatibility concern here, we've >>>>> never had a FreeBSD release that reports "arm64" for "uname -m". >>>>> There's no reason for us to prefer "arm64" if everyone else uses >>>>> "aarch64." Also, having arm64 for uname -m and aarch64 for uname >>>>> -p seems a bit odd. >>>> >>>> I would assume uname -m would be "arm", not "arm64". Unless there >>>> are fundamental platform differences you are baking in somehow, >>>> which I don't know. >>> >>> arm would be a pleasing outcome, but looking at his WIP tree, it >>> looks like it would be possible, but rather inconvenient to merge the >>> arm64 bits back under arm and make them conditional. >> >> They are two different architectures. They don't share any assembly, >> and the exception handling is different, both in exception types and >> number of modes/levels. >> >> Along with this they only sort of share the special registers. The >> method to access them is different, on 32-bit it is via a coprocessor >> call where on 64-bit there is an instruction to get them by name. >> >> We may be able to share some of the new pmap code, however it would >> need a lot of work as the virtual memory layout is different and it is >> likely we will need to handle 64k granules on arm64 in the future. >> Because of this any sharing there would need to be handled carefully. >> >> The interrupt controller and timer drivers will be shared, but these >> are both devices and maybe they should be moved under sys/dev. > > Yea, rather inconvenient :) Yes. For my own interest, how similar are 32-bit ARMv8 and 64-bit ARMv8? Is 32-bit ARMv8 just the same as 32-bit ARMv7? It seems weird that the architectures would be that disjoint, but, if they are, I agree it makes sense more to treat them like ia64 and i386 than a conventional 32-bit/64-bit port. >> The two architectures will share very little code or headers. An ARMv8 >> core may be able to execute either 32 or 64-bit code (both are optional >> so either one or both options will be enabled) so there is a case for >> use to handle cc -m32, but I don't feel this is enough justification to >> merge two otherwise different architectures just because they were >> designed by the same company. > > We support -m32 on x86 where we have amd64 and i386 MACHINE values > and directories today. I’m not sure how having either aarch64/aarch64 or > arm64/aarch64 instead of arm/aarch64 would preclude -m32 from working. It just makes it harder since /usr/include/machine isn't shared. There were a lot of hacks required to make that functional on x86, which you'd have to repeat. -Nathan
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