Re: [PATCH] KVM: emulator: Only allow VMCALL/VMMCALL trapped by #UD

["Yang, Sheng" <sheng.yang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On Monday 07 January 2008 18:43:52 Avi Kivity wrote:
> Yang, Sheng wrote:
> > On Monday 07 January 2008 17:22:43 Avi Kivity wrote:
> >> Yang, Sheng wrote:
> >>>> I have a vague plan for improving decode; basically extend the decode
> >>>> tables to add group decoding.  We add a bit to opcode_table and
> >>>> twobyte_table that is set for all instructions which need group
> >>>> decoding.  When the bit is set, the rest of the value in opcode_table
> >>>> is interpreted as an index (together with modrm_reg) into a new
> >>>> group_table, so we can have different decoding for such instructions.
> >>>
> >>> I also have tried to propose a table for Grp opcode, but can't find a
> >>> easy way. Using the rest of the value in opcode_table is a good idea,
> >>> but I'm afraid the same value for different group exists, e.g.
> >>> 0x82(Grp1) and 0xc0 (Grp2) got the same value as: ByteOp | DstMem |
> >>> SrcImm | ModRM. If we add more factors to this, it would become unclear
> >>> and more tricky, the table also may become larger...
> >>>
> >>> Currently, if we want to using group_table, I think the straightforward
> >>> way is better: another big "switch"... The only exception is 1a, and we
> >>> may use 0 instead of it.
> >>
> >> Not sure what you mean.  I thought of adding code like
> >>
> >>
> >>     if (c->d & Group) {
> >>           c->group = c->d & GroupMask;
> >
> > I meant the c->d & GroupMask is not sufficient to indicate different
> > group. For example, 0x82(Grp1) and 0xc0(Grp2) have same c->d & GroupMask
> > = ByteOp | DstMem | SrcImm | ModRM.
>
> I now see the source of confusion...
>
> >>           // fetch modrm_reg
> >>           c->d = group_table[c->group * 8 + modrm_reg];
> >
> > In this case, how can you deal with c->group =  ByteOp | DstMem | SrcImm
> > | ModRM, and modrm_reg = 6. Is it a XOR or nothing?
>
> In this case, we would have
>
>    opcode_table[0x82] == Group | Grp1  (c->group == Grp1 == 1)
>    opcode_table[0xc0] == Group | Grp2  (c->group == Grp2 == 2)
>    group_table[Grp1 * 8 + 6] == ByteOp | DstMem | SrcImm | ModRM
>    group_table[Grp2 * 8 + 0] == ByteOp | DstMem | SrcImm | ModRM
>
> If Group is set in opcode_table, the rest of the value is the group
> number, with the reg part used to pick the final decode flags.

Faint... 

Yeah, I think that's pretty good. Sorry for misunderstanding...

-- 
Thanks
Yang, Sheng

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