Re: [Xen-devel] [PATCH v3 2/3] x86/emulate: add support of emulating SSE2 instruction {, v}movd mm, r32/m32 and {, v}movq mm, r64

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.08.16 at 16:55, <mdontu@xxxxxxxxxxxxxxx> wrote:
> On Monday 01 August 2016 17:48:33 Mihai Donțu wrote:
>> On Monday 01 August 2016 07:43:20 Jan Beulich wrote:
>> > > > > Your suggestion makes sense, but I'm starting to doubt my initial
>> > > > > patch. :-) I'm testing "movq xmm1, xmm1" and noticing that it takes 
>> > > > > the
>> > > > > GPR-handling route and I can't seem to be able to easily prevent it
>> > > > > with !(rex_prefix & REX_B), as rex_prefix == 0 and vex.b == 1. I need
>> > > > > to take a harder look at how that class of instructions is coded.    
>> > > > >   
>> > > > 
>> > > > You obviously need to distinguish the two kinds of register sources/
>> > > > destinations: GPRs need suitable re-writing of the instruction (without
>> > > > having looked at the most recent version of the patch yet I btw doubt
>> > > > converting register to memory operands is the most efficient model),
>> > > > while MMs, XMMs, and YMMs can retain their register encoding.    
>> > > 
>> > > Regarding efficiency, I'm not married with the approach I've proposed.
>> > > If you can give me a few more hints, I can give it a try.    
>> > 
>> > I'd rather pick a fixed register and update the regs->... field from that
>> > after the stub was executed. E.g. using rAX and treating it just like a
>> > return value of the "call". But maybe I'm imagining this easier than it
>> > really is; as an alternative I'd then suggest really following what Andrew
>> > said - use a pointer into regs->, not mmvalp. But (as said in the review
>> > mail) you'd then have the problem of the missing zero-extension for
>> > writes to 32-bit GPRs  
>> 
>> I thought that by re-using (hijacking, really) mmvalp, the patch will
>> look less intrusive and thus not add too much to an already complex
>> code.
>> 
>> Assuming I'll just pass to the stub "a"(ea.reg), would it be a good
>> idea to just zero-out the 64bit register before that? It does not
>> appear to be any instructions that write just the low dword. Or am I
>> misunderstanding the zero-extension concept?
> 
> Just to be sure I'm making myself understood, ea.reg contains the
> output of decode_register() which, in turn, returns a pointer in regs.

Hmm, now that you say that maybe I got confused be the expression
you used: If, rather than doing a cast on the address of mmvalp, you
could get away with a simply assignment (and maybe a cast on the
rvalue), then I suppose your code might not be as wrong as it seemed
to me.

Jan

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