[PATCH v2] KVM: arm64: Add tracepoint for MMIO accesses where ISV==0

[PATCH v2] KVM: arm64: Add tracepoint for MMIO accesses where ISV==0

[Oliver Upton]

It is a pretty well known fact that KVM does not support MMIO emulation
without valid instruction syndrome information (ESR_EL2.ISV == 0). The
current kvm_pr_unimpl() is pretty useless, as it contains zero context
to relate the event to a vCPU.

Replace it with a precise tracepoint that dumps the relevant context
so the user can make sense of what the guest is doing.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
v1: https://lore.kernel.org/kvmarm/ZToh7pNhz1zzQw6C@linux.dev/
v1 -> v2:
 - Add a tracepoint to improve debuggability (Marc)

Tested with the page_fault_test selftest, which does pre-indexed
accesses outside of a memslot.

 arch/arm64/kvm/mmio.c      |  4 +++-
 arch/arm64/kvm/trace_arm.h | 25 +++++++++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

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),

base-commit: 6465e260f48790807eef06b583b38ca9789b6072
-- 
2.42.0.820.g83a721a137-goog

Re: [PATCH v2] KVM: arm64: Add tracepoint for MMIO accesses where ISV==0

[Zenghui Yu]

On 2023/10/27 4:53, Oliver Upton wrote:
> It is a pretty well known fact that KVM does not support MMIO emulation
> without valid instruction syndrome information (ESR_EL2.ISV == 0). The
> current kvm_pr_unimpl() is pretty useless, as it contains zero context
> to relate the event to a vCPU.
> 
> Replace it with a precise tracepoint that dumps the relevant context
> so the user can make sense of what the guest is doing.
> 
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Acked-by: Zenghui Yu <yuzenghui@huawei.com>

Re: [PATCH v2] KVM: arm64: Add tracepoint for MMIO accesses where ISV==0

[Marc Zyngier]

On Thu, 26 Oct 2023 21:53:06 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> It is a pretty well known fact that KVM does not support MMIO emulation
> without valid instruction syndrome information (ESR_EL2.ISV == 0). The
> current kvm_pr_unimpl() is pretty useless, as it contains zero context
> to relate the event to a vCPU.
> 
> Replace it with a precise tracepoint that dumps the relevant context
> so the user can make sense of what the guest is doing.
> 
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Acked-by: Marc Zyngier <maz@kernel.org>

Thanks for going the extra mile on this!

	M.

-- 
Without deviation from the norm, progress is not possible.