linux-kernel - Re: [PATCH v4 00/24] ILP32 for ARM64

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Message-ID: <E769538B-CE45-4FF3-AD2C-21B30069F0B9@caviumnetworks.com>
Date:	Thu, 16 Apr 2015 11:33:49 +0000
From:	"Pinski, Andrew" <Andrew.Pinski@...iumnetworks.com>
To:	"Dr. Philipp Tomsich" <philipp.tomsich@...obroma-systems.com>
CC:	Catalin Marinas <catalin.marinas@....com>,
	Alexander Graf <agraf@...e.de>,
	Andreas Kraschitzer <andreas.kraschitzer@...obroma-systems.com>,
	"Arnd Bergmann" <arnd@...db.de>, Andreas Schwab <schwab@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Pinski <apinski@...ium.com>,
	Kumar Sankaran <ksankaran@....com>,
	Benedikt Huber <benedikt.huber@...obroma-systems.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Christoph Muellner <christoph.muellner@...obroma-systems.com>
Subject: Re: [PATCH v4 00/24] ILP32 for ARM64





> On Apr 16, 2015, at 4:19 AM, Dr. Philipp Tomsich <philipp.tomsich@...obroma-systems.com> wrote:
> 
> Just for the record (and to avoid anyone wasting their time on what’s available
> today): we are migrating this over to option (a) now, even though we would
> prefer to see option (b) implemented.
> 
> If we get a consensus on (b) in the next couple of days, we’ll redo things for
> option (b). If not, we will have an implementation for option (a) available that
> we can hopefully all agree on merging.

I don't think either a or b are good in the long run. There are only a few places where long should be 32bit rather than 64bit. The non-time_t field of timespec is the only one I can think of. The rest are valid and good idea to stay as 64bit. Including the limits. I think this whole discussion should have happened over a year ago. And I thought c was decided back then. I had even implemented a originally and then asked to move over to c. So I am a bit upset now we are making this kind of huge changes to the abi a year after the original posting of the patch. 

Also why does it takes over a year to accept patches into the linux kernel when it takes much less time to make huge changes into gcc (pointer plus is an example which took only a few months to accept and it was an infrastructure change and this is not even an infrastructure change). 

Thanks,
Andrew


> 
> Best,
> Phil.
> 
>> On 16 Apr 2015, at 13:03, Catalin Marinas <catalin.marinas@....com> wrote:
>> 
>> On Thu, Apr 16, 2015 at 12:25:36AM +0200, Alexander Graf wrote:
>>> We've just started to bootstrap openSUSE for ILP32 with the non-final
>>> abi. However, keep in mind that at least for us bootstrapping is a
>>> manual process. So changing syscall numbers means we'll need to go
>>> through the manual process again.
>>> 
>>> So if you need any help on getting you an environment that allows you to
>>> build LTP with whatever syscall abi people come up with, feel free to
>>> poke Andreas or me.
>> 
>> Thanks for the offer. It's great to see a full distro being built (even
>> though no commitment).
>> 
>> But I'm a bit worried that people started using these patches and we
>> haven't agreed on the ABI yet (well, for a while we thought that the x32
>> approach was fine until I've seen objections from others).
>> 
>> Maybe a quick poll on the options, ignoring syscall number details (we
>> go for the generic ones) or syscall tables sharing:
>> 
>> a) AArch32-like ILP32 ABI, 32-bit time_t, 32-bit __kernel_long_t
>>  (POSIX-compliant)
>> b) Future-proof ILP32 ABI, 64-bit time_t, 32-bit __kernel_long_t (as
>>  seen by the user) (POSIX-compliant)
>> c) LP64-like ILP32 ABI, 64-bit time_t, 64-bit __kernel_long_t
>>  (non-POSIX-compliant)
>> 
>> Option (a) is the easiest from the kernel perspective and has the
>> highest chance of not breaking legacy AArch32 applications.
>> 
>> Option (b) is more future looking (beyond 2038) but it introduces a
>> third ABI in the kernel (incompatible with both compat and native).
>> There is also a risk that legacy applications assume a 32-bit time_t.
>> 
>> Option (c) is pretty much what we currently have in these patches. While
>> many syscalls are native LP64, it doesn't fully solve pointers in
>> structures shared between user and kernel (ioctl being one of the
>> affected areas)
>> 
>> I can't tell how bad the impact of non-POSIX compliance is. If this is
>> essential, between (a) and (b) I'm more in favour of (a) as the easiest
>> for both kernel and user. If we were to start a new ABI from scratch
>> without legacy applications, I would have definitely gone for (b) but
>> I'm worried about legacy applications still not fully working with this
>> option while adding more maintenance burden in the kernel.
>> 
>> -- 
>> Catalin
> 
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