[Devel] [vz7.7 PATCH] x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested

Roman Kagan rkagan at virtuozzo.com
Tue Feb 13 12:49:05 MSK 2018

From: Vitaly Kuznetsov <vkuznets at redhat.com>

I was investigating an issue with seabios >= 1.10 which stopped working
for nested KVM on Hyper-V. The problem appears to be in
handle_ept_violation() function: when we do fast mmio we need to skip
the instruction so we do kvm_skip_emulated_instruction(). This, however,
depends on VM_EXIT_INSTRUCTION_LEN field being set correctly in VMCS.
However, this is not the case.

Intel's manual doesn't mandate VM_EXIT_INSTRUCTION_LEN to be set when
EPT MISCONFIG occurs. While on real hardware it was observed to be set,
some hypervisors follow the spec and don't set it; we end up advancing
IP with some random value.

I checked with Microsoft and they confirmed they don't fill

Fix the issue by doing instruction skip through emulator when running

Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
Suggested-by: Radim Krčmář <rkrcmar at redhat.com>
Suggested-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets at redhat.com>
Acked-by: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar at redhat.com>
[rkagan: the problem pertains to ESXi as well]
(cherry picked from commit d391f1207067268261add0485f0f34503539c5b0)
[rkagan: adjusted for vz7.7]
VZ-Bug: #PSBM-81462
Signed-off-by: Roman Kagan <rkagan at virtuozzo.com>
 arch/x86/kvm/vmx.c | 18 ++++++++++++++++--
 arch/x86/kvm/x86.c |  3 ++-
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index d94946a07de6..8ab2b3d42adc 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -5934,9 +5934,23 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
 	gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
 	if (!kvm_io_bus_write(vcpu->kvm, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
-		skip_emulated_instruction(vcpu);
-		return 1;
+		/*
+		 * Doing kvm_skip_emulated_instruction() depends on undefined
+		 * behavior: Intel's manual doesn't mandate
+		 * occurs and while on real hardware it was observed to be set,
+		 * other hypervisors (namely Hyper-V) don't set it, we end up
+		 * advancing IP with some random value. Disable fast mmio when
+		 * running nested and keep it for real hardware in hope that
+		 * VM_EXIT_INSTRUCTION_LEN will always be set correctly.
+		 */
+		if (!static_cpu_has(X86_FEATURE_HYPERVISOR)) {
+			skip_emulated_instruction(vcpu);
+			return 1;
+		} else
+			return x86_emulate_instruction(vcpu, gpa, EMULTYPE_SKIP,
+						       NULL, 0) == EMULATE_DONE;
 	ret = handle_mmio_page_fault(vcpu, gpa, true);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c3103bdd2d37..ddb3c2529ca8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5503,7 +5503,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 		 * handle watchpoints yet, those would be handled in
 		 * the emulate_ops.
-		if (kvm_vcpu_check_breakpoint(vcpu, &r))
+		if (!(emulation_type & EMULTYPE_SKIP) &&
+		    kvm_vcpu_check_breakpoint(vcpu, &r))
 			return r;
 		ctxt->interruptibility = 0;

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