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

[Sebastian Ott]

On Wed, 1 May 2024, Oliver Upton wrote:
> 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.

Ugh, I didn't think about KVM_ARM_VCPU_INIT being called more than once.

But in that case don't we loose the changes done to hcr_el2 in the current
code? E.g.:

void kvm_init_sysreg(struct kvm_vcpu *vcpu)
{
..
	if (!kvm_has_feat(kvm, ID_AA64ISAR0_EL1, TLB, OS))
		vcpu->arch.hcr_el2 |= HCR_TTLBOS;
..
}

static inline void vcpu_reset_hcr(struct kvm_vcpu *vcpu)
{
	vcpu->arch.hcr_el2 = HCR_GUEST_FLAGS;
..
}

Thanks,
Sebastian