Re: [Qemu-devel] [Qemu-ppc] How to add my implementation of the fmadds instruction to QEMU

[Programmingkid]

On Sep 29, 2016, at 2:19 PM, Alex Bennée wrote:

> 
> Programmingkid <address@hidden> writes:
> 
>> On Sep 29, 2016, at 12:17 AM, David Gibson wrote:
>> 
>>> On Tue, Sep 27, 2016 at 09:58:02AM -0700, Peter Maydell wrote:
>>>> On 27 September 2016 at 09:51, G 3 <address@hidden> wrote:
>>>>> The problem with your reasoning is you assume this instruction has to be
>>>>> 100% correctly implemented. That every single "corner-case" has to be
>>>>> accounted for.
>>>> 
>>>> For upstream QEMU we've already made this design decision --
>>>> emulation accuracy comes first, and speed is secondary.
>>>> That's why we implement this the way we do.
>>> 
>>> I think there is a way you could get both speed and accuracy, but it's
>>> a huge project:
>>> 
>>> You'd need to add full float awareness to TCG - so floating point TCG
>>> values and floating point operations as tcp micro-ops, defined
>>> according to IEEE semantics.  Then you'd need to rewrite the TCG
>>> frontends in terms of those new ops, at least for target CPUs close
>>> enough to IEEE semantics for that to work.  And you'd need to rewrite
>>> the TCG backends to implement those fp ops in terms of host cpu fp
>>> instructions .. at least when the host has fp behaviour close enough
>>> to IEEE to make that work, with a fallback to soft float when that's
>>> not the case.
>> 
>> Interesting idea. Do you think we would see a large enough increase in speed
>> to justify this project?
> 
> It really depends on workload. If you want to run lots of encoding/audio
> workloads in TCG guests it is certainly something that could be
> improved.
> 
> As others have pointed out however it is a fairly big project.
> 
> --
> Alex Bennée

Alex Bennée? I was just watching your KVM video about MTTCG! Small world. 

I so want audio to play correctly in a PowerPC Mac OS guest. So this
project might be necessary. 

If it is a fairly big project, then I will need to map it out some more.
I've made a mind map of what I know so far. It is attached to this email.
Let me know if you can think of anything to add.

http://i.imgur.com/MYkiKGx.png