From mboxrd@z Thu Jan 1 00:00:00 1970 From: catalin.marinas@arm.com (Catalin Marinas) Date: Fri, 4 Jul 2014 09:24:37 +0100 Subject: Android and compatibility with deprecated armv7 instructions In-Reply-To: References: <20140702163923.GJ410@sirena.org.uk> <20140702170141.GH24879@arm.com> <20140703104135.GA28175@arm.com> <20140703170558.GZ32514@n2100.arm.linux.org.uk> <20140703173226.GC17372@arm.com> <20140703224753.GB21766@n2100.arm.linux.org.uk> Message-ID: <20140704082437.GA16404@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Fri, Jul 04, 2014 at 08:08:05AM +0100, Ard Biesheuvel wrote: > On 4 July 2014 00:47, Russell King - ARM Linux wrote: > > On Thu, Jul 03, 2014 at 11:16:16PM +0100, M?ns Rullg?rd wrote: > >> Will Deacon writes: > >> > >> > On Thu, Jul 03, 2014 at 06:05:58PM +0100, Russell King - ARM Linux wrote: > >> >> On Thu, Jul 03, 2014 at 05:22:30PM +0100, Grant Likely wrote: > >> >> > So, no. I completely reject any notion that breaking existing apps is > >> >> > okay. If we're going to say that v8 still supports 32-bit apps, then > >> >> > it has to be all of v7, not just the 'good' bits. Nor do I think > >> >> > saying "it's just a bunch of games" justifies anything. We're kernel > >> >> > engineers. Applications are applications and we don't break userspace. > >> >> > Period. > >> >> > >> >> +1 on all points above. I'd go further - if we're going to say that v8 > >> >> still supports 32-bit apps, that covers at least v6 *as well*. > >> > > >> > We've never pretended to support anything other than ARMv8 in the compat > >> > layer. uname even reports this in the machine name. > >> > > >> > If people are suddenly so concerned about *full* compatibility with an ARMv7 > >> > kernel, that needs a lot more than just SWP emulation: > >> > > >> > - Alignment fixups for ldm/stm > >> > >> No ARM variant ever supported unaligned ldm/stm. > > > > Quite right but that's not the point being discussed. Please note that > > the sentence says "with an ARMv7 *kernel*" - we are not talking about > > the architecture there. > > > > So, what's more to the point is that on 32-bit ARM userspace under Linux, > > we _have_ supported it since early 2000 up to present. It's not currently > > supported on 64-bit ARM running Linux, even when running a 32-bit binary > > in userspace. > > > > Ergo, it's a user visible ABI change, one which we don't know whether it > > matters. In all probability, it doesn't because (hopefully) no one ever > > does unaligned LDMs/STMs - I think it would require hand-crafted assembly, > > at which point you're talking about optimising something, and you'd be > > silly to do it as it would invoke the alignment fault handling which > > would be slow. > > > > Well, if something like this > > struct bar { > long l[4]; > }; > > void foo(struct bar *dst, struct bar const *src) > { > *dst = *src; > } > > produces this: > > foo: > @ args = 0, pretend = 0, frame = 0 > @ frame_needed = 0, uses_anonymous_args = 0 > @ link register save eliminated. > mov ip, r0 > ldmia r1, {r0, r1, r2, r3} > stmia ip, {r0, r1, r2, r3} > bx lr > > won't it take just a single cast from some unaligned char* to struct > bar* to trigger this? Is this even allowed by the C ABI? The compiler generates the LDMs because function foo() gets a struct pointer which is guaranteed to be aligned. -- Catalin