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


On Wed, Jun 28, 2017 at 2:02 AM, Kugan Vivekanandarajah
<kugan.vivekanandarajah@linaro.org> wrote:
> Hi Ramana,
>
> On 27 June 2017 at 18:01, Ramana Radhakrishnan
> <ramana.radhakrishnan@foss.arm.com> wrote:
>> On 27/06/17 02:20, 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
>>
>>
>> Is that because your compiler has defaulted to -mpc-relative-literal-loads
>> because it has the workaround enabled by default ? I'm curious as to why
>> others haven't seen this issue.
>>
>
> If TARGET_FIX_ERR_A53_843419 is selected, compiler defaults to
> -mpc-relative-literal-loads unless we explicitly specify
> -mno-pc-relative-literal-loads. Linaro toolchain is built with
> TARGET_FIX_ERR_A53_843419.

That explains why we haven't been hit by this issue in our builds of
SPEC2017 even though I don't think we've done any lto builds
recently.,
>
> This linking of TARGET_FIX_ERR_A53_843419 and
> -mpc-relative-literal-loads  should now be relaxed since the kernel
> explicitly uses -mpc-relative-literal-loads.
>
> This 1MiB issue should be very rarely seen even before you fixed it.
>

This particular issue maybe, but the original patch was put in because
we had a number of users complaining about functions > 1MiB especially
with autogenerated code.

regards
Ramana

> Thanks,
> Kugan
>
>
>> regards
>> Ramana


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