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

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

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.

-- 
error compiling committee.c: too many arguments to function


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