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Subject[PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS
Microsoft pointed out privately to me that KVM's handling of
KVM_FAST_MMIO_BUS is invalid. Using skip_emulation_instruction is invalid
in EPT misconfiguration vmexit handlers, because neither EPT violations
nor misconfigurations are listed in the manual among the VM exits that
set the VM-exit instruction length field.

While physical processors seem to set the field, this is not architectural
and is just a side effect of the implementation. I couldn't convince
myself of any condition on the exit qualification where VM-exit
instruction length "has" to be defined; there are no trap-like VM-exits
that can be repurposed; and fault-like VM-exits such as descriptor-table
exits provide no decoding information. So I don't really see any elegant
way to fix it except by disabling KVM_FAST_MMIO_BUS, which means virtio
1 will go slower.

Adding a hypercall or MSR write that does a fast MMIO write to a physical
address would do it, but it adds hypervisor knowledge in virtio, including
CPUID handling. So it would be pretty ugly in the guest-side implementation,
but if somebody wants to do it and the virtio side is acceptable to the
virtio maintainers, I am okay with it.

Cc: Michael S. Tsirkin <>
Cc: Radim Krčmář <>
Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
Signed-off-by: Paolo Bonzini <>
arch/x86/kvm/vmx.c | 5 -----
1 file changed, 5 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 375dca24cf42..b3eaeb20670d 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6320,11 +6320,6 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
gpa_t gpa;

gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
- if (!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
- trace_kvm_fast_mmio(gpa);
- return kvm_skip_emulated_instruction(vcpu);
- }
ret = handle_mmio_page_fault(vcpu, gpa, true);
vcpu->arch.gpa_available = true;
if (likely(ret == RET_MMIO_PF_EMULATE))
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