[RFC v2 PATCH 7/7] arm64: KVM: Enable selective trapping of TLB instructions

Christoffer Dall [off-list ref] writes:

On Mon, Sep 05, 2016 at 05:31:37PM +0100, Punit Agrawal wrote:

quoted

The TTLB bit of Hypervisor Control Register (HCR_EL2) controls the
trapping of guest TLB maintenance instructions. Taking the trap requires
a switch to the hypervisor and is an expensive operation.

Enable selective trapping of guest TLB instructions when the associated
perf trace event is enabled for a specific virtual machine.

Signed-off-by: Punit Agrawal <redacted>
Cc: Christoffer Dall <redacted>
Cc: Marc Zyngier <redacted>
---
 arch/arm64/kvm/perf_trace.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/arch/arm64/kvm/perf_trace.c b/arch/arm64/kvm/perf_trace.c
index 8bacd18..f26da1d 100644
--- a/arch/arm64/kvm/perf_trace.c
+++ b/arch/arm64/kvm/perf_trace.c

@@ -17,6 +17,8 @@
 #include <linux/kvm_host.h>
 #include <linux/trace_events.h>
 
+#include <asm/kvm_emulate.h>
+
 typedef int (*perf_trace_callback_fn)(struct kvm *kvm, bool enable);
 
 struct kvm_trace_hook {

@@ -24,7 +26,37 @@ struct kvm_trace_hook {
 	perf_trace_callback_fn setup_fn;
 };
 
+static int tlb_invalidate_trap(struct kvm *kvm, bool enable)
+{
+	int i;
+	struct kvm_vcpu *vcpu;
+
+	/*
+	 * Halt the VM to ensure atomic update across all vcpus (this
+	 * avoids racy behaviour against other modifications of
+	 * HCR_EL2 such as kvm_toggle_cache/kvm_set_way_flush).
+	 */
+	kvm_arm_halt_guest(kvm);
+	kvm_for_each_vcpu(i, vcpu, kvm) {
+		unsigned long hcr = vcpu_get_hcr(vcpu);
+
+		if (enable)
+			hcr |= HCR_TTLB;
+		else
+			hcr &= ~HCR_TTLB;
+
+		vcpu_set_hcr(vcpu, hcr);
+	}
+	kvm_arm_resume_guest(kvm);
+
+	return 0;
+}
+
 static struct kvm_trace_hook trace_hook[] = {
+	{
+		.key = "kvm_tlb_invalidate",

is this key defined elsewhere?  If not, I think the name is ambiguous,
because it's unclear if this means 'kvm the subsystem' or 'a kvm guest',
by looking purely at the string.

The key is the name of the tracepoint that the callback is executed
against. Here it is the tracepoint defined in Patch 6/7.

I've now added a comment to the definition of "struct kvm_trace_hook" to
explain what the key is.

quoted

+		.setup_fn = tlb_invalidate_trap,
+	},
 	{ },
 };
 
-- 
2.8.1

Otherwise looks ok to me.

-Christoffer
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