X86EMUL_PROPAGATE_FAULT

DISCUSSION Matteo 3 participants 3 messages May 5, 2011 – May 15, 2011 KVM

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Matteo

author

2011-05-05 15:05 UTC #0 lore raw

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<------------------------------------------------

Any hints helping in addressing this error is highly welcome,

Thank you in advance,

Matteo

Avi Kivity

2011-05-09 08:50 UTC #1 lore raw

Quoted 39 lines · Matteo

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<-----------------------------------------------

Quoted 8 lines

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.

Signature · 2 lines

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

kvm

2011-05-15 22:31 UTC #2 lore raw

As regards this issue,
I don't think it's a guest OS bug
since running the same OS without the "RET" emulation I get no error.
If this bug is related to the ret emulation code, could you please provide a fix?

thanks,

Quoted 51 lines · Matteo

Quoted 39 lines
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<-----------------------------------------------
Quoted 8 lines
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.