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

Oliver Upton
2023-10-26T20:53:27Z

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