Re: [AARCH64] Disable pc relative literal load irrespective of TARGET_FIX_ERR_A53_84341

[Kugan Vivekanandarajah <kugan dot vivekanandarajah at linaro dot org>]

Hi James,

On 29 August 2017 at 21:31, James Greenhalgh <james.greenhalgh@arm.com> wrote:
> On Tue, Jun 27, 2017 at 11:20:02AM +1000, Kugan Vivekanandarajah wrote:
>> https://gcc.gnu.org/ml/gcc-patches/2016-03/msg00614.html  added this
>> workaround to get kernel building with when TARGET_FIX_ERR_A53_843419
>> is enabled.
>>
>> This was added to support building kernel loadable modules. In kernel,
>> when CONFIG_ARM64_ERRATUM_843419 is selected, the relocation needed
>> for ADRP/LDR (R_AARCH64_ADR_PREL_PG_HI21 and
>> R_AARCH64_ADR_PREL_PG_HI21_NC are removed from the kernel to avoid
>> loading objects with possibly offending sequence). Thus, it could only
>> support pc relative literal loads.
>>
>> However, the following patch was posted to kernel to add
>> -mpc-relative-literal-loads
>> http://www.spinics.net/lists/arm-kernel/msg476149.html
>>
>> -mpc-relative-literal-loads is unconditionally added to the kernel
>> build as can be seen from:
>> https://github.com/torvalds/linux/blob/master/arch/arm64/Makefile
>>
>> Therefore this patch removes the hunk so that applications like
>> SPECcpu2017's 521/621.wrf can be built (with LTO in this case) without
>> -mno-pc-relative-literal-loads
>>
>> Bootstrapped and regression tested on aarch64-linux-gnu with no new regressions.
>>
>> Is this OK for trunk?
>
> Hi Kugan,
>
> I'm struggling a little to convince myself that this is correct. I think
> the argument is that this was a workaround for one very particular issue
> with the linux kernel module loader, which hard faults by refusing to
> handle these relocations when in a workaround mode for Erratum 843419.

Yes.

> Most of these relocations won't occur because the kernel builds with
> -mcmodel=large, but literals would always use a small model style
> adrp/load, unless we were using -mpc-relative-literal-loads . So, this
> workaround enabled -mpc-relative-literal-loads always when we were in
> a workaround mode, thus allowing the kernel loader to continue.
>
> The argument for removing it then, is that with the kernel now always using
> -mpc-relative-literal-loads there is no reason for this workaround to stay
> in place. The linkers which we will use will apply the workaround if needed.

Yes.

> Testcases and a detailed problem report of the build failures you had seen in
> 521.wrf would have helped me get closer to understanding this, and made
> review substantially easier.

Sorry for not being clear with this. Unfortunately 521.wrf  was
showing this error with LTO so I could not reproduce with a small
enough test case.

> Am I on the right track?
>
> If so, this is OK for trunk. If not, please can you expand on what is going
> on in this patch.

Thanks for the review.
Kugan

>
> Thanks,
> James
>
>
>>
>> Thanks,
>> Kugan
>>
>> gcc/testsuite/ChangeLog:
>>
>> 2017-06-27  Kugan Vivekanandarajah  <kuganv@linaro.org>
>>
>>     * gcc.target/aarch64/pr63304_1.c: Remove-mno-fix-cortex-a53-843419.
>>
>> gcc/ChangeLog:
>>
>> 2017-06-27  Kugan Vivekanandarajah  <kuganv@linaro.org>
>>
>>     * config/aarch64/aarch64.c (aarch64_override_options_after_change_1):
>>     Disable pc relative literal load irrespective of TARGET_FIX_ERR_A53_84341
>>     for default.
>
>