Errata-1315703 WA disabled in Neoverse N1 SDP

In Trusted Firmware-A, all erratum are disabled by default including the 1315703. If we want any erratum to be enabled, then we have to explicitly enable them in the platform Makefile.

The N1SDP stack disables the workaround for Erratum 1315703 by default, so that the N1 CPU performance in N1SDP better reflects that of released versions of the N1 for software that does not require mitigation for Spectre Variant 4.

N1SDP uses N1 version r1p0, which is affected by Erratum 1315703, which is fixed in N1 r3p1. The workaround for r1p0 disables the CPU performance feature of bypassing of stores by younger loads. This can significantly affect performance. The Erratum is classified “Cat A (Rare)” and requires a specific sequence of events to occur.

Disabling this CPU performance feature is also the mitigation for Spectre Variant 4 (CVE-2018-3639). On CPUs that provide the PSTATE.SBSS feature, the OS selectively applies the mitigation only to programs that require it, leaving the performance of other programs unaffected. However, N1 r1p0 does not have the PSTATE.SBSS feature (which is introduced in N1 r3p1), and TF-A does not provide the interface to dynamically disable the CPU performance feature. Therefore, applying the workaround penalizes ALL software running on N1SDP, including those that do not require the mitigation.

Disabling Errata-1315703 is meant for performance evaluation purposes ONLY and should not be used for software that requires a seccomp computing environment.