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Subject[PATCH v2 2/2] KVM: x86: On emulation failure, convey the exit reason to userspace
Date
Should instruction emulation fail, include the VM exit reason in the
emulation_failure data passed to userspace, in order that the VMM can
report it as a debugging aid when describing the failure.

Suggested-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: David Edmondson <david.edmondson@oracle.com>
---
arch/x86/include/asm/kvm_host.h | 2 ++
arch/x86/kvm/vmx/vmx.c | 5 +----
arch/x86/kvm/x86.c | 22 +++++++++++++---------
include/uapi/linux/kvm.h | 7 +++++++
4 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 0ee580c68839..2e411e26e40e 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1630,6 +1630,8 @@ extern u64 kvm_mce_cap_supported;
int kvm_emulate_instruction(struct kvm_vcpu *vcpu, int emulation_type);
int kvm_emulate_instruction_from_buffer(struct kvm_vcpu *vcpu,
void *insn, int insn_len);
+void kvm_prepare_emulation_failure_exit(struct kvm_vcpu *vcpu,
+ bool instruction_bytes);

void kvm_enable_efer_bits(u64);
bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d9a4d6cf6406..4fb240204c2c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -5367,10 +5367,7 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)

if (vmx->emulation_required && !vmx->rmode.vm86_active &&
vcpu->arch.exception.pending) {
- vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
- vcpu->run->internal.suberror =
- KVM_INTERNAL_ERROR_EMULATION;
- vcpu->run->internal.ndata = 0;
+ kvm_prepare_emulation_failure_exit(vcpu, false);
return 0;
}

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 17468d983fbd..bf30b445b65d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7452,7 +7452,8 @@ void kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip)
}
EXPORT_SYMBOL_GPL(kvm_inject_realmode_interrupt);

-static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
+void kvm_prepare_emulation_failure_exit(struct kvm_vcpu *vcpu,
+ bool instruction_bytes)
{
struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
u32 insn_size = ctxt->fetch.end - ctxt->fetch.data;
@@ -7463,7 +7464,7 @@ static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
run->emulation_failure.ndata = 0;
run->emulation_failure.flags = 0;

- if (insn_size) {
+ if (insn_size && instruction_bytes) {
run->emulation_failure.ndata = 3;
run->emulation_failure.flags |=
KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES;
@@ -7473,7 +7474,14 @@ static void prepare_emulation_failure_exit(struct kvm_vcpu *vcpu)
memcpy(run->emulation_failure.insn_bytes,
ctxt->fetch.data, insn_size);
}
+
+ run->emulation_failure.ndata = 4;
+ run->emulation_failure.flags |=
+ KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON;
+ run->emulation_failure.exit_reason =
+ static_call(kvm_x86_get_exit_reason)(vcpu);
}
+EXPORT_SYMBOL_GPL(kvm_prepare_emulation_failure_exit);

static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)
{
@@ -7489,16 +7497,14 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu, int emulation_type)

if (kvm->arch.exit_on_emulation_error ||
(emulation_type & EMULTYPE_SKIP)) {
- prepare_emulation_failure_exit(vcpu);
+ kvm_prepare_emulation_failure_exit(vcpu, true);
return 0;
}

kvm_queue_exception(vcpu, UD_VECTOR);

if (!is_guest_mode(vcpu) && static_call(kvm_x86_get_cpl)(vcpu) == 0) {
- vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
- vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
- vcpu->run->internal.ndata = 0;
+ kvm_prepare_emulation_failure_exit(vcpu, false);
return 0;
}

@@ -12092,9 +12098,7 @@ int kvm_handle_memory_failure(struct kvm_vcpu *vcpu, int r,
* doesn't seem to be a real use-case behind such requests, just return
* KVM_EXIT_INTERNAL_ERROR for now.
*/
- vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
- vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
- vcpu->run->internal.ndata = 0;
+ kvm_prepare_emulation_failure_exit(vcpu, false);

return 0;
}
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index d9e4aabcb31a..863195371272 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -282,6 +282,7 @@ struct kvm_xen_exit {

/* Flags that describe what fields in emulation_failure hold valid data. */
#define KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES (1ULL << 0)
+#define KVM_INTERNAL_ERROR_EMULATION_FLAG_EXIT_REASON (1ULL << 1)

/* for KVM_RUN, returned by mmap(vcpu_fd, offset=0) */
struct kvm_run {
@@ -404,6 +405,12 @@ struct kvm_run {
__u64 flags;
__u8 insn_size;
__u8 insn_bytes[15];
+ /*
+ * The "exit reason" extracted from the
+ * VMCS/VMCB that was the cause of attempted
+ * emulation.
+ */
+ __u64 exit_reason;
} emulation_failure;
/* KVM_EXIT_OSI */
struct {
--
2.30.2
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