Disabling cache

[Everaldo-Gomes]

Hi,
I'm trying to disable the cache before Bao initializes the VMs, but by the time the code responsible for this is executed, Bao freezes.

To disable it, I followed these steps

The responsible register is this

When disabling the bit 2 (C), it freezes.

This occurs when using the SCTLR_EL2 register, but changing the SCTLR_EL1 does not freeze, but the value in it does not change.

I'd like to know, what I can do to solve this, and prevent the VMs from caching any data.

Platform: Raspberry Pi 4b
Demo: Dual-guest Linux+FreeRTOS

[josecm]

Sorry for the delay @Everaldo-Gomes.

If you are not flushing the caches before disabling them for the hypervisor (i.e., in SCTLR_EL2) this kind of behavior should be expected because the data in memory will be inconsistent with that in the cache. Any kind of unexpected behavior should be expected (pun intended) at this point.

[josecm]

However, to prevent VMs to cache data you can simply mark the second-stage page table entries as uncachable by fiddling with the memory attributes field. You can do that by changing the PTE_VM_FLAGS macro.

[Everaldo-Gomes]

All flags in PTE_VM_FLAGS are set to OFF, such as PTE_MEMATTR_NRML_OWBC, PTE_MEMATTR_NRML_IWBC, PTE_SH_NS, and PTE_S2AP_RW.

But the flag PTE_AF has the value (1LL << 10). Should I change this value to something else?

[josecm]

I am not sure about the encoding, i.e., if they should be "off". They should be set such that the hypervisor forces guest memory to be non-cacheable independently of how the guest configures its page tables. Check the second entry in this table of the Arm ARM:

~

[Everaldo-Gomes]

Understood. Thank you!