Messages in this thread |  | | Subject | Re: [PATCH] arm/arm64: smccc: Use xN for arm64 register constraints with clang | From | Greg Hackmann <> | Date | Thu, 22 Mar 2018 16:19:42 -0700 |
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On 03/22/2018 03:44 PM, Matthias Kaehlcke wrote: > El Thu, Mar 22, 2018 at 10:26:18PM +0000 Nick Desaulniers ha dit: > >> Note that a patch in this form has previously been implemented by: >> >> Andrey Konovalov <andreyknvl@google.com>: >> https://gist.github.com/xairy/ee11682ea86044a45c0291c528cd936f >> >> and another by: >> >> Greg Hackmann <ghackmann@google.com>: >> https://android-review.googlesource.com/c/kernel/common/+/645181 >> >> If you used either as a reference, you may want to credit them with a >> `Suggested-by:` in the commit message. > > Not really, but I think I prefer Greg's version over mine and might > use it in a respin if nobody raises objections.
NAK. There's a reason I didn't send my change upstream.
As Marc pointed out (https://lkml.org/lkml/2018/3/16/987), the "r" prefix tells gcc to pick the appropriate register width. "x" makes it unconditionally use the entire 64-bit register width. Just swapping out one for the other changes the macro's semantics.
Unfortunately since this was breaking builds in android-4.14 and we didn't have an immediate-term fix, I bit the bullet and added the above commit -- but *only* as a short-term workaround. For the one caller we currently have in 4.14.y, gcc was using the entire 64-bit width for all its inputs anyway, so "r" vs. "x" didn't make a difference. But that might not be true if/when someone introduces other SMCCC 1.1 callers.
Unfortunately I don't see a better way to deal with this than waiting for clang to support "r"-style constraints on ARM64.
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