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

Avi Kivity
2012-05-07T10:18:05Z

On 05/07/2012 01:12 PM, Joerg Roedel wrote:
> On Sun, May 06, 2012 at 11:21:52AM +0300, Avi Kivity wrote:
> > > 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).
>
> Right, thats a better solution. How about the attached patch? The zf
> check shouldn't be necessary anymore because the generated assembly uses
> dst.val as input and output so writeback shouldn't do anything wrong.
> The bsr and bsf unittests all pass again with this patch.
>
> Joerg
>
> From e9262f18e90111d32b584084c0b5564cbd728d65 Mon Sep 17 00:00:00 2001
> From: Joerg Roedel <joerg.roedel@amd.com>
> Date: Mon, 7 May 2012 12:05:28 +0200
> Subject: [PATCH] KVM: X86: convert bsf/bsr instructions to
> emulate_2op_SrcV_nobyte()
>
> The instruction emulation for bsrw is broken in KVM because
> the code always uses bsr with 32 or 64 bit operand size for
> emulation. Fix that by using emulate_2op_SrcV_nobyte() macro
> to use guest operand size for emulation.
>

It looks fine. Do you know what triggered this regression? (for
figuring out if it's 3.4 material)