Re: [PATCH] KVM: X86: Remove stale values from ctxt->memop before emulation

Avi Kivity
2012-05-06T08:21:52Z

On 05/04/2012 07:14 PM, Joerg Roedel wrote:
> When instruction decoding begins there could be stale values
> in the ctxt->memop structure. This causes problems when an
> instruction is emulated with more op-bytes then the guest
> wants (like the bsr instruction which is always emulated
> with 4 or 8 op-bytes).
>
> The stale value in this structure causes the unit-test for
> the bsrw instruction to fail. Initialize the memop.val with
> 0 to prevent such bugs (an alternative fix could be to
> always emulate instructions with the number of op-bytes
> requested by the guest).
>
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index d4bf50c..1b516ec 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -3937,6 +3937,7 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
> struct opcode opcode;
>
> ctxt->memop.type = OP_NONE;
> + ctxt->memop.val = 0;
> ctxt->memopp = NULL;
> ctxt->_eip = ctxt->eip;
> ctxt->fetch.start = ctxt->_eip;

This only works for long sized values - it doesn't initialize val64 on
i386, for example. So I think it's better to change bsr (and family) to
use emualte_2op_SrcV_nobyte() instead (which has the added benefit of
using the same values as the processor for the "undefined" bits).