Re: [Minios-devel] [UNIKRAFT PATCHv4 22/43] plat/kvm: Allow access to floating-point and Advanced SIMD registers

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 23/07/18 11:02, Wei Chen wrote:
> > -----Original Message-----
> > From: Yuri Volchkov <yuri.volchkov@xxxxxxxxx>
> > Sent: 2018年7月23日 17:52
> > To: Wei Chen <Wei.Chen@xxxxxxx>; Sharan Santhanam <sharan.santhanam@xxxxxxxxx>;
> > minios-devel@xxxxxxxxxxxxxxxxxxxx; simon.kuenzer@xxxxxxxxx
> > Cc: Kaly Xin <Kaly.Xin@xxxxxxx>; Julien Grall <Julien.Grall@xxxxxxx>; nd
> > <nd@xxxxxxx>; Dave P Martin <Dave.Martin@xxxxxxx>
> > Subject: Re: [Minios-devel] [UNIKRAFT PATCHv4 22/43] plat/kvm: Allow access to
> > floating-point and Advanced SIMD registers
> >
> > Hi Wei
> >
> > > As Unikraft is not a kernel.
> > That is not completely true. That is matter of terminology. We still
> > separate "kernel" (or "core" if you like) code from the
> > "application". Even though it is melted together into one address space.
> >
> > Or do you mean it is not a kernel because it runs as a qemu process?
> >  From that perspective yes.. but that is still an OS (virtualized
> > though). If linux runs in KVM it has a kernel anyways..
> I said it's not a kernel is from the user's view. If you transfer nginx
> to Unikraft-nginx. I don't think the user will consider Unikraft-nginx
> is kernel. But from technology view, yes, LIBOS still have the features
> of a kernel has.
>
> > I think it would be best to disable floating point for "core" part. And
> > if a specific application (e.g. DPDK) needs it, only that application's
> > code should be build with floating point support.
> No, it's not realistic, the FP feature is a CPU feature. Except we want to
> do a switch for core and application, We can't enable it for application
> part only. What we can do is enable FP&SIMD registers access for application
> but disable FP&SIMD registers for core part through GCC flags. But this means
> we need two independent code for common functions. For example, you have to
> compile uk_print_nofp for core with -mgeneric-registers-only, and compile
> Uk_print for applications.
I agree with Yuri here. You don't want the "core" to have floating point 
enabled. If you do enable floating point, you will need to save/restore 
them on entry/exit of a "syscall" and exception.
This will add latency on interrupt because of the number of registers to 
save. I am not even mentioning SVE here... Even Linux, which support 
only FP in the userspace, has been looking at limiting the save/restore 
of FP by using a Lazy solution (i.e the FP are enabled on demand).
FP support is not going to be trivial. Overall I think it would be 
better if we keep FP support off for now. This could be implemented in a 
follow-up series.
Cheers,

--
Julien Grall

_______________________________________________
Minios-devel mailing list
Minios-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/minios-devel