Re: [Minios-devel] [UNIKRAFT RFC PATCH 2/5] plat/kvm: arm64: Enable the fp/simd at the starting point

[Jia He <jiakernel2@xxxxxxxxx>]

Hi Julien

On 2019/12/20 15:50, Julien Grall wrote:
> On 20/12/2019 01:14, Justin He wrote:
> > Hi Julien
> Hello,
>
> > > -----Original Message-----
> > > From: Julien Grall <julien@xxxxxxx>
> > > Sent: Thursday, December 19, 2019 10:35 PM
> > > To: Justin He <Justin.He@xxxxxxx>; minios-devel@xxxxxxxxxxxxxxxxxxxx;
> > > Simon Kuenzer <simon.kuenzer@xxxxxxxxx>; Santiago.Pagani@xxxxxxxxx
> > > Cc: Felipe Huici <felipe.huici@xxxxxxxxx>; Kaly Xin <Kaly.Xin@xxxxxxx>;
> > > Julien Grall <Julien.Grall@xxxxxxx>; Sharan.Santhanam@xxxxxxxxx
> > > Subject: Re: [Minios-devel] [UNIKRAFT RFC PATCH 2/5] plat/kvm: arm64:
> > > Enable the fp/simd at the starting point
> > >
> > > Hi,
> > >
> > > On 19/12/2019 14:27, Jia He wrote:
> > > > Write the sys reg to enable the fp/simd feature, otherwise it will
> > > > cause floating point/simd exception when touching q0-q31.
> > > In your first patch you say FPSIMD will be available for application
> > > only. But here, you will allow Unikraft itself to use it.
> > >
> > > So don't you want to turn this only when jumping to the application code?
> > Yes, this might be more graceful.
> > But there is a corner case in libukdebug of Unikraft.
> > If we removed -mgeneral-regs-only, some printf* family functions (especially
> > those have variable length parameters, .e.g va_start/end) will be compiled to
> > use q0-q31.
> I have already had a lenghty discussion in the past with Wei Chen (see [1]) 
> about it.
> If you tell GCC to enable floating point for the kernel, then your exception 
> path (e.g interrupt) may also use floating. Therefore you would also have to 
> save/restore them at *every* exception to prevent any corruption.
Maybe let me clarify it more clear. Sorry if it caused some misunderstanding.

In principle, unkraft kernel part can't use fpsimd, but 
-mgeneral-registers-only can be global
only. It mean currently I merely can enable/disable -mgeneral-registers-only 
globally for
every unikraft source file, even for application part and newlib part.


So to resovle it perfectly, seems we need to use different build flag to 
kernel, application/newlib
part? E.g.

1. for unikraft context switch .S file, it shoultn't use 
-mgeneral-registers-only because we need
to save/restore v0-v31 registers

2. for other unikraft kernel source file, we need to use 
-mgeneral-registers-only to forbid the
potential abuse.

3. for application and newlib source file, we need to use 
-mgeneral-registers-only

What do you think of it? thanks


---
Cheers,
Justin (Jia He)


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