I've run into an issue with exception handling on the Raspberry Pi 3 in 32-bit mode (config rpi_3_32b) and the ODroid XU4 (config odroid-xu3). When testing the "exception undefined" command, instead of printing out register info and resetting the CPU like it's supposed to, U-Boot just hangs.
I noticed that on both of these boards, U-Boot runs in hypervisor mode (HYP) because both the Pi's Cortex-A53 and the ODroid's Cortex-A7 support virtualization extensions. This causes several issues with the ARMv7 exception handling code, which assumes that exceptions are taken in EL1.
(All file names and line numbers referenced below are respect to the v2021.10 release)
The first issue running in HYP mode is that the vector table base address is not set correctly. In arch/arm/cpu/armv7/start.S L78 and arch/arm/lib/relocate.S L45, the VBAR register is loaded with the vector table base address. However, exceptions taken to EL2 use the address in the HVBAR register, not VBAR.
The next issue is that the get_bad_stack macro (arch/arm/lib/vectors.S) uses "movs pc, lr" to return from the exception into supervisor mode, but this is an illegal instruction in HYP mode as it was replaced by "eret". Also, the lr register does not hold the exception return address (which is used to determine the PC before the exception), since exceptions taken to EL2 store the return address in the new ELR_hyp register instead. I'm also not sure in reading this code why there's a switch to supervisor mode...
So to handle an exception to EL2 properly, an instruction like "msr ELR_hyp, lr" needs to be assembled, and this requires "-march=armv7ve" in the CC flags, not "-march=armv7-a".
I found it quite surprising that this didn't work correctly. I'd be happy to submit a patch for it, and could use some guidance. To change the CC flag to the correct architecture, it seems like introducing a CONFIG_CPU_V7VE may be the right approach. Then I'd say that for builds with this config, there should be two separate exception vector tables, one for EL1, which would be left as it currently is, and a new one for EL2, with this table loaded into HVBAR.
I'd appreciate your thoughts, thanks.