[PATCH 0/8] ARM: clean up PC-relative arithmetic

[Russell King - ARM Linux]

On Thu, Aug 04, 2016 at 12:11:23PM +0200, Ard Biesheuvel wrote:
> On 4 August 2016 at 12:03, Russell King - ARM Linux
> <linux at armlinux.org.uk> wrote:
> > On Thu, Aug 04, 2016 at 11:54:25AM +0200, Ard Biesheuvel wrote:
> >> On 4 August 2016 at 11:49, Russell King - ARM Linux
> >> <linux at armlinux.org.uk> wrote:
> >> > On Thu, Aug 04, 2016 at 09:17:04AM +0200, Ard Biesheuvel wrote:
> >> >> On 3 August 2016 at 20:17, Russell King - ARM Linux
> >> >> <linux at armlinux.org.uk> wrote:
> >> >> > I don't buy that argument, sorry, and the argument is actually wrong.
> >> >> > No, we're _not_ letting the linker do the calculations for us, we're
> >> >> > letting the linker do _some_ of the calculation, but not all.
> >> >> >
> >> >> > What you're replacing the above with is stuff like (I guess, because
> >> >> > I've no idea what this :pc_g0: notation is):
> >> >> >
> >> >> >         add     rX, pc, #(sym - . - 8) & 0xff
> >> >> >         add     rX, rX, #(sym - . - 4) & 0xff00
> >> >> >         add     rX, rX, #(sym - .) & 0xff0000
> >> >> >
> >> >> > which I think is a more complex (and less obvious) way to calculate it.
> >> >> > It's also buggy when we end up with a relative offset greater than 16MB,
> >> >> > which we have in multi-zImage kernels.
> >> >> >
> >> >>
> >> >> Even if you think this is a more complex way to calculate it, at least
> >> >> it is encapsulated in a single macro instead of having similar but not
> >> >> identical open coded instances all over the place.
> >> >
> >> > ... and, it may come as a shocker, but I don't have a problem with
> >> > that.
> >> >
> >> >> As for the range: the ldr/str variants have 28 bits of range (2x
> >> >> scaled 8 bit immediate for the adds and a single unscaled 12 bit
> >> >> immediate for the ldr/str). The adr variant has 26 bits (3x scaled
> >> >> immediate counting from bit 2) range for word aligned symbols, which
> >> >> gives us +/- 64 MB, which should be plenty. The only pathological
> >> >> outlier is allyesconfig, but that uses Thumb2 anyway.
> >> >
> >> > Our existing code allows for a range of the full address space - the only
> >> > thing it relies upon is that the literal data is placed within reach of
> >> > the code - which it will be, because it's always placed near the code
> >> > which is using it.
> >> >
> >> >> The relocations documented here
> >> >> http://infocenter.arm.com/help/topic/com.arm.doc.ihi0044f/IHI0044F_aaelf.pdf
> >> >
> >> > Right, so it's an EABI thing, and I guess you haven't tested OABI
> >> > builds, where I suspect these relocations aren't supported.
> >> >
> >>
> >> I suppose that's a fair point. But then, I'm only 40 so I am too young
> >> to remember this OABI stuff anyway. Does it require GCC 2.95 from your
> >> toolchain museum?
> >
> > I'm sorry, but that's really no excuse, we're of similar ages, so...
> > <expletive deleted>.
> 
> Just countering the unnecessary sarcasm ... :-)

There was nothing sarcastic about my reply to you - everything I said
was deadly serious.

-- 
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