RE: [PATCH v6 07/10] x86, mpx: decode MPX instruction to get bound violation information

[Ren, Qiaowei]

On 2014-06-20, H. Peter Anvin wrote:
> On 06/19/2014 10:04 AM, Dave Hansen wrote:
>> 
>> Could you please support this position with some data?  I'm a bit
>> skeptical that instruction decoding is going to be a
>> performance-critical path.
>> 
>> I also don't see the extra field that you talked about in the
>> previous thread?  What's the extra field?  I see a 'limit' vs.
>> 'length', but you don't use 'length' at all, so I think you can use
>> it instead, or at least union it.
>> 
>> I've taken a quick stab at trying to consolidate things.  I think I
>> may have screwed up this:
>> 
>> 	insn->limit = MAX_MPX_INSN_SIZE - bytes;
>> 
>> Qiaowei, is there anything fundamentally broken with what I've got here?
>> 
> 
Firstly instructions will be got from user pointer stored in 'ip', and then validate_next() will use 'limit' to make sure that next sizeof(t) bytes can be on the same instruction.

As hpa said, generic decoder, including struct insn and implementation of decoding, is very heavyweight because it has to. So MPX specific decoding should be better choice.

> So I encouraged Qiaowei to do a limited special-purpose decoder,
> simply because the glue to use the generic decoder was almost as
> large.  I am overall not a huge fan of using the generic decoder in
> constrained situation, because the generic decoder is very heavyweight
> not just in terms of performance but in terms of interface -- because it has to.
> 
Thanks,
Qiaowei