[PATCH v2 02/12] KVM: arm64: nv: Sync nested timer state with FEAT_NV2

Marc Zyngier
2024-12-17T14:23:26Z

Emulating the timers with FEAT_NV2 is a bit odd, as the timers
can be reconfigured behind our back without the hypervisor even
noticing. In the VHE case, that's an actual regression in the
architecture...

Co-developed-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
arch/arm64/kvm/arch_timer.c | 44 ++++++++++++++++++++++++++++++++++++
arch/arm64/kvm/arm.c | 3 +++
include/kvm/arm_arch_timer.h | 1 +
3 files changed, 48 insertions(+)

diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
index 1215df5904185..ee5f732fbbece 100644
--- a/arch/arm64/kvm/arch_timer.c
+++ b/arch/arm64/kvm/arch_timer.c
@@ -905,6 +905,50 @@ void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
kvm_timer_blocking(vcpu);
}

+void kvm_timer_sync_nested(struct kvm_vcpu *vcpu)
+{
+ /*
+ * When NV2 is on, guest hypervisors have their EL1 timer register
+ * accesses redirected to the VNCR page. Any guest action taken on
+ * the timer is postponed until the next exit, leading to a very
+ * poor quality of emulation.
+ */
+ if (!is_hyp_ctxt(vcpu))
+ return;
+
+ if (!vcpu_el2_e2h_is_set(vcpu)) {
+ /*
+ * A non-VHE guest hypervisor doesn't have any direct access
+ * to its timers: the EL2 registers trap (and the HW is
+ * fully emulated), while the EL0 registers access memory
+ * despite the access being notionally direct. Boo.
+ *
+ * We update the hardware timer registers with the
+ * latest value written by the guest to the VNCR page
+ * and let the hardware take care of the rest.
+ */
+ write_sysreg_el0(__vcpu_sys_reg(vcpu, CNTV_CTL_EL0), SYS_CNTV_CTL);
+ write_sysreg_el0(__vcpu_sys_reg(vcpu, CNTV_CVAL_EL0), SYS_CNTV_CVAL);
+ write_sysreg_el0(__vcpu_sys_reg(vcpu, CNTP_CTL_EL0), SYS_CNTP_CTL);
+ write_sysreg_el0(__vcpu_sys_reg(vcpu, CNTP_CVAL_EL0), SYS_CNTP_CVAL);
+ } else {
+ /*
+ * For a VHE guest hypervisor, the EL2 state is directly
+ * stored in the host EL1 timers, while the emulated EL0
+ * state is stored in the VNCR page. The latter could have
+ * been updated behind our back, and we must reset the
+ * emulation of the timers.
+ */
+ struct timer_map map;
+ get_timer_map(vcpu, &map);
+
+ soft_timer_cancel(&map.emul_vtimer->hrtimer);
+ soft_timer_cancel(&map.emul_ptimer->hrtimer);
+ timer_emulate(map.emul_vtimer);
+ timer_emulate(map.emul_ptimer);
+ }
+}
+
/*
* With a userspace irqchip we have to check if the guest de-asserted the
* timer and if so, unmask the timer irq signal on the host interrupt
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index a102c3aebdbc4..fa3089822f9f3 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1228,6 +1228,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
if (unlikely(!irqchip_in_kernel(vcpu->kvm)))
kvm_timer_sync_user(vcpu);

+ if (vcpu_has_nv(vcpu))
+ kvm_timer_sync_nested(vcpu);
+
kvm_arch_vcpu_ctxsync_fp(vcpu);

/*
diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
index fd650a8789b91..6e3f6b7ff2b22 100644
--- a/include/kvm/arm_arch_timer.h
+++ b/include/kvm/arm_arch_timer.h
@@ -98,6 +98,7 @@ int __init kvm_timer_hyp_init(bool has_gic);
int kvm_timer_enable(struct kvm_vcpu *vcpu);
void kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu);
void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu);
+void kvm_timer_sync_nested(struct kvm_vcpu *vcpu);
void kvm_timer_sync_user(struct kvm_vcpu *vcpu);
bool kvm_timer_should_notify_user(struct kvm_vcpu *vcpu);
void kvm_timer_update_run(struct kvm_vcpu *vcpu);
--
2.39.2