Re: [PATCH v2 2/6] KVM: arm64: unify trap setup code

From: Oliver Upton
Date: Wed May 01 2024 - 02:51:29 EST


On Fri, Apr 26, 2024 at 12:49:46PM +0200, Sebastian Ott wrote:
> There are 2 functions to set up traps via HCR_EL2:

nitpick: these functions *calculate* the trap values, but do not
actually set them up. HCR_EL2 doesn't get written to until further down
the line on KVM_RUN.

> + if (cpus_have_final_cap(ARM64_HAS_STAGE2_FWB)) {
> + vcpu->arch.hcr_el2 |= HCR_FWB;
> + } else {
> + /*
> + * For non-FWB CPUs, we trap VM ops (HCR_EL2.TVM) until M+C
> + * get set in SCTLR_EL1 such that we can detect when the guest
> + * MMU gets turned on and do the necessary cache maintenance
> + * then.
> + */
> + vcpu->arch.hcr_el2 |= HCR_TVM;
> + }

It seems to me like calling this once for the lifetime of a vCPU will
break non-FWB behavior.

Like the comment suggests, these traps are needed to catch the moment
the S1 MMU is turned on and do cache maintenance to make sure D$ agrees
with what the guest was doing before enabling the MMU.

KVM_ARM_VCPU_INIT resets SCTLR_EL1, but it seems we'd miss setting
HCR_TVM in that case.

--
Thanks,
Oliver