On Sep 29, 2016, at 6:36 PM, Alex Bennée wrote: > > Programmingkid <programmingk...@gmail.com> writes: > >> On Sep 29, 2016, at 2:19 PM, Alex Bennée wrote: >> >>> >>> Programmingkid <programmingk...@gmail.com> 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 <programmingk...@gmail.com> 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 > > While I appreciate your target is PPC I think if you are going to > suggest any core floating point TCGOps you will need to survey the > behaviour of the FPUs on all (or at least the most common) TCG targets > and go for instructions that behave the same across a broad range of > targets. > > I think if we were to introduce this into the code base we would need to > have a decent range of test cases. I'm talking about making sure we > exercise the whole range of behaviour: > > - min/max rounding behaviour > - handling of denormalisation > - signalling and non-signalling NaN behaviour > - exception generation > > Testing is going to be very important for confidence.
Thank you very much for this list. I will see what I can come up with.
