[PATCH v2 03/12] KVM: arm64: nv: Publish emulated timer interrupt state in the in-memory state

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

With FEAT_NV2, the EL0 timer state is entirely stored in memory,
meaning that the hypervisor can only provide a very poor emulation.

The only thing we can really do is to publish the interrupt state
in the guest view of CNT{P,V}_CTL_EL0, and defer everything else
to the next exit.

Only FEAT_ECV will allow us to fix it, at the cost of extra trapping.

Suggested-by: Chase Conklin <chase.conklin@arm.com>
Suggested-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
arch/arm64/kvm/arch_timer.c | 21 +++++++++++++++++++++
arch/arm64/kvm/arm.c | 2 +-
2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
index ee5f732fbbece..8bff913ed1264 100644
--- a/arch/arm64/kvm/arch_timer.c
+++ b/arch/arm64/kvm/arch_timer.c
@@ -441,11 +441,30 @@ void kvm_timer_update_run(struct kvm_vcpu *vcpu)
regs->device_irq_level |= KVM_ARM_DEV_EL1_PTIMER;
}

+static void kvm_timer_update_status(struct arch_timer_context *ctx, bool level)
+{
+ /*
+ * Paper over NV2 brokenness by publishing the interrupt status
+ * bit. This still results in a poor quality of emulation (guest
+ * writes will have no effect until the next exit).
+ *
+ * But hey, it's fast, right?
+ */
+ if (is_hyp_ctxt(ctx->vcpu) &&
+ (ctx == vcpu_vtimer(ctx->vcpu) || ctx == vcpu_ptimer(ctx->vcpu))) {
+ unsigned long val = timer_get_ctl(ctx);
+ __assign_bit(__ffs(ARCH_TIMER_CTRL_IT_STAT), &val, level);
+ timer_set_ctl(ctx, val);
+ }
+}
+
static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
struct arch_timer_context *timer_ctx)
{
int ret;

+ kvm_timer_update_status(timer_ctx, new_level);
+
timer_ctx->irq.level = new_level;
trace_kvm_timer_update_irq(vcpu->vcpu_id, timer_irq(timer_ctx),
timer_ctx->irq.level);
@@ -471,6 +490,8 @@ static void timer_emulate(struct arch_timer_context *ctx)
return;
}

+ kvm_timer_update_status(ctx, should_fire);
+
/*
* If the timer can fire now, we don't need to have a soft timer
* scheduled for the future. If the timer cannot fire at all,
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index fa3089822f9f3..bda905022df40 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1228,7 +1228,7 @@ 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))
+ if (is_hyp_ctxt(vcpu))
kvm_timer_sync_nested(vcpu);

kvm_arch_vcpu_ctxsync_fp(vcpu);
--
2.39.2