KVM does not currently honor the trap flag when emulating instructions that cause VM exits. This is observable from guest userspace, try stepping on a CPUID instruction in gdb in a KVM guest. The program will stop two instructions after CPUID. To fix this, add kvm_skip_emulated_instruction that calls kvm_x86_ops->skip_emulated_instruction and then kvm_vcpu_check_singlestep to check for RFLAGS.TF. Patch 4 does this. To handle both the guest setting TF and the KVM_GUESTDBG_SINGLESTEP cases we need to be able to indicate to callees that an exit to userspace is required. Patches 1-3 are largely plumbing to make this possible. Traps triggered by task switch instructions require some additional handling and are not implemented. KVM_GUESTDBG_SINGLESTEP traps can be squashed by certain instructions which also trigger userspace exits, such as HALT, MOV CR8, and IO instructions. I believe (although I have not tested) that KVM will simply generate another trap on the next instruction, so this is no worse than the current behavior. Patch 5 fixes an existing bug where kvm_vcpu_check_singlestep clears RFLAGS.TF when generating a singlestep trap. Bare silicon does not do this. These patches only fix this issue for VMX, and the handful of SVM cases that call the shared kvm_complete_insn_gp. I don't have AMD silicon to test on. Since v1: Patch 3: KVM: VMX: Move skip_emulated_instruction out of nested_vmx_check_vmcs12 - No longer creates a separate skip_emulated_instruction_no_trap variant. Patch 4: KVM: x86: Add kvm_skip_emulated_instruction and use it. - Instead the existing skip_emulated_instruction continues to not check the TF flag. - kvm_skip_emulated_instruction is added to skip_emulated_instruction and then deal with any singlestep traps that should be generated. - The patch 5 from v1 is effectively subsumed by this patch. Patch 5: KVM: x86: Do not clear RFLAGS.TF when a singlestep trap occurs. - New in this version, fixes an existing bug visible to the kvm-unit-tests test I wrote for this patch series. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
