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* [PATCH v2 0/5] KVM: VMX: Fix single stepping with emulated instructions
@ 2016-11-29 20:40 Kyle Huey
  2016-11-29 20:40 ` [PATCH v2 1/5] KVM: x86: Add a return value to kvm_emulate_cpuid Kyle Huey
                   ` (5 more replies)
  0 siblings, 6 replies; 9+ messages in thread
From: Kyle Huey @ 2016-11-29 20:40 UTC (permalink / raw)
  To: Paolo Bonzini, Radim Krčmář, Thomas Gleixner,
	Ingo Molnar, H. Peter Anvin, x86, Joerg Roedel
  Cc: kvm, linux-kernel

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.

^ permalink raw reply	[flat|nested] 9+ messages in thread

end of thread, other threads:[~2016-12-19 20:55 UTC | newest]

Thread overview: 9+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2016-11-29 20:40 [PATCH v2 0/5] KVM: VMX: Fix single stepping with emulated instructions Kyle Huey
2016-11-29 20:40 ` [PATCH v2 1/5] KVM: x86: Add a return value to kvm_emulate_cpuid Kyle Huey
2016-11-29 20:40 ` [PATCH v2 2/5] KVM: VMX: Reorder some skip_emulated_instruction calls Kyle Huey
2016-11-29 20:40 ` [PATCH v2 3/5] KVM: VMX: Move skip_emulated_instruction out of nested_vmx_check_vmcs12 Kyle Huey
2016-12-19 20:54   ` David Matlack
2016-11-29 20:40 ` [PATCH v2 4/5] KVM: x86: Add kvm_skip_emulated_instruction and use it Kyle Huey
2016-11-29 20:40 ` [PATCH v2 5/5] KVM: x86: Do not clear RFLAGS.TF when a singlestep trap occurs Kyle Huey
2016-11-30 18:41 ` [PATCH v2 0/5] KVM: VMX: Fix single stepping with emulated instructions Radim Krčmář
2016-11-30 18:52   ` Kyle Huey

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