On 05/05/2011 06:05 PM, Matteo wrote:
Hello to everybody,
I am working on KVM version 2.6.38
and I'm facing a new problem on an emulated instruction
whose implementation is already in kvm.
The error shows up after the emulation of the "RET" opcode (C3 Byte
Opcode).
When trying to emulate the instruction
at the address loaded after the pop instruction made by the RET
I get an X86EMUL_PROPAGATE_FAULT error due to a gpa == UNMAPPED_GVA
as you can see in the following debug trace:
---------------8<-------------------------------------------------
x86_decode_insn:2705 -> Starting New Instruction Decode
x86_decode_insn:2709 -> c->eip = ctxt->eip = 3226138255
x86_decode_insn:2759 -> Opcode -> c3
x86_decode_insn:2928 -> Decode and fetch the source operand
x86_decode_insn:2931 -> SrcNone
x86_decode_insn:3015 -> Decode and fetch the second source operand
x86_decode_insn:3018 -> Src2None
x86_decode_insn:3044 -> Decode and fetch the destination operand
x86_decode_insn:3089 -> ImplicitOps
x86_decode_insn:3092 -> No Destination Operand
x86_emulate_instruction:4458 -> Returned from x86_decode_insn with r = 0
x86_emulate_insn:3194 -> starting special_insn...
x86_emulate_insn:3196 -> c->eip = 3226138256
x86_emulate_insn:3565 -> starting writeback...
writeback:1178 -> c->eip = 2147483648
x86_emulate_instruction:4538 -> Return from x86_emulate_insn with code
r = 0
---------------------------8<-----------------------------------------------
So the next instruction will be emulated reading the opcode with
eip=2147483648 as stated before
but the emulation fails with the following debug trace
---------------------------8<-----------------------------------------------
x86_decode_insn:2705 -> Starting New Instruction Decode
x86_decode_insn:2709 -> c->eip = ctxt->eip = 2147483648
x86_decode_insn:2757 -> Read opcode from eip
kvm_read_guest_virt_helper:3724 -> gpa == UNMAPPED_GVA return
X86EMUL_PROPAGATE_FAULT
do_fetch_insn_byte:573 -> ops->fetch returns an error
---------------------------8<------------------------------------------------
The instruction has returned to an EIP that is outside RAM, so kvm is
unable to fetch the next instruction. This is likely due to a bug (in
kvm or the guest) that has occurred much earlier.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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