Re: [PATCH] KVM: arm64: Stop printing about MMIO accesses where ISV==0

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, Oct 25, 2023 at 09:41:01AM +0100, Marc Zyngier wrote:
> On Wed, 25 Oct 2023 09:25:07 +0100,
> Oliver Upton <oliver.upton@linux.dev> wrote:
> > 
> > On Wed, Oct 25, 2023 at 09:04:58AM +0100, Marc Zyngier wrote:
> > 
> > [...]
> > 
> > > While I totally agree that this *debug* statement should go, we should
> > > also replace it with something else.
> > > 
> > > Because when you're trying to debug a guest (or even KVM itself),
> > > seeing this message is a sure indication that the guest is performing
> > > an access outside of memory. The fact that KVM tries to handle it as
> > > MMIO is just an implementation artefact.
> > > 
> > > So I'd very much welcome a replacement tracepoint giving a bit more
> > > information, such as guest PC, IPA being accessed, load or store. With
> > > that, everybody wins.
> > 
> > Aren't we already covered by the kvm_guest_fault tracepoint? Userspace
> > can filter events on ESR to get the faults it cares about. I'm not
> > against adding another tracepoint, but in my experience kvm_guest_fault
> > has been rather useful for debugging any type of guest fault.
> 
> That tracepoint is one of the most triggered, and sifting through this
> is a painful experience. If we go down that road, adding a bit of
> extra documentation (pointed to from the KVM_RUN entry) and an example
> filter script would be most useful.

Eh, I'd rather write kernel code than documentation, and I think you
knew that too ;-)

How do you feel about this:

diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
index 3dd38a151d2a..200c8019a82a 100644
--- a/arch/arm64/kvm/mmio.c
+++ b/arch/arm64/kvm/mmio.c
@@ -135,6 +135,9 @@ int io_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa)
 	 * volunteered to do so, and bail out otherwise.
 	 */
 	if (!kvm_vcpu_dabt_isvalid(vcpu)) {
+		trace_kvm_mmio_nisv(*vcpu_pc(vcpu), kvm_vcpu_get_esr(vcpu),
+				    kvm_vcpu_get_hfar(vcpu), fault_ipa);
+
 		if (test_bit(KVM_ARCH_FLAG_RETURN_NISV_IO_ABORT_TO_USER,
 			     &vcpu->kvm->arch.flags)) {
 			run->exit_reason = KVM_EXIT_ARM_NISV;
@@ -143,7 +146,6 @@ int io_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa)
 			return 0;
 		}
 
-		kvm_pr_unimpl("Data abort outside memslots with no valid syndrome info\n");
 		return -ENOSYS;
 	}
 
diff --git a/arch/arm64/kvm/trace_arm.h b/arch/arm64/kvm/trace_arm.h
index 8ad53104934d..c18c1a95831e 100644
--- a/arch/arm64/kvm/trace_arm.h
+++ b/arch/arm64/kvm/trace_arm.h
@@ -136,6 +136,31 @@ TRACE_EVENT(kvm_mmio_emulate,
 		  __entry->vcpu_pc, __entry->instr, __entry->cpsr)
 );
 
+TRACE_EVENT(kvm_mmio_nisv,
+	TP_PROTO(unsigned long vcpu_pc, unsigned long esr,
+		 unsigned long far, unsigned long ipa),
+	TP_ARGS(vcpu_pc, esr, far, ipa),
+
+	TP_STRUCT__entry(
+		__field(	unsigned long,	vcpu_pc		)
+		__field(	unsigned long,	esr		)
+		__field(	unsigned long,	far		)
+		__field(	unsigned long,	ipa		)
+	),
+
+	TP_fast_assign(
+		__entry->vcpu_pc		= vcpu_pc;
+		__entry->esr			= esr;
+		__entry->far			= far;
+		__entry->ipa			= ipa;
+	),
+
+	TP_printk("ipa %#016lx, esr %#016lx, far %#016lx, pc %#016lx",
+		  __entry->ipa, __entry->esr,
+		  __entry->far, __entry->vcpu_pc)
+);
+
+
 TRACE_EVENT(kvm_set_way_flush,
 	    TP_PROTO(unsigned long vcpu_pc, bool cache),
 	    TP_ARGS(vcpu_pc, cache),

-- 
Thanks,
Oliver