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

[Julien Grall <julien.grall@xxxxxxx>]

(+ Dave)

On 09/07/18 10:47, Wei Chen wrote:
> Hi, Julien,
Hi Wei,

> > -----Original Message-----
> > From: Julien Grall <julien.grall@xxxxxxx>
> > Sent: 2018年7月8日 6:17
> > To: Wei Chen <Wei.Chen@xxxxxxx>; minios-devel@xxxxxxxxxxxxxxxxxxxx;
> > simon.kuenzer@xxxxxxxxx
> > Cc: Kaly Xin <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> > Subject: Re: [Minios-devel] [UNIKRAFT PATCHv4 22/43] plat/kvm: Allow access to
> > floating-point and Advanced SIMD registers
> >
> > Hi,
> >
> > On 07/06/2018 10:03 AM, Wei Chen wrote:
> > > On Arm64, when we want to print out some message, the va_list will
> > > use the SIMD&FP registers (like q0, q1) to store parameters. So, we
> > > have to disable the trap of accessing floating-point and Advanced SIMD
> > > registers to make va_list use SIMD&FP registers properly.
> > I don't think this is true. Xen and Linux don't use SIMD&FP registers as
> > this would require to context switch at every entry. Yet we are able to
> > use va_list without SIMD&FP registers.
> This is true, because Linux and Xen doesn't support float print.
> Unikraft kernel just a part of an application, its print library need
> To support float. Just like nolibc, when GCC compiling this library,
> it doesn't care you are running EL1 or EL0, the generated code will
> access q0, q1. When you're running this code in EL1 without SIMD&FP,
> system crash.
When will q0, q1 will be accessed? Is it only when floating point will 
be printed?
Also, IIUC, the kernel code will be compiled with floating point, right? 
If so, it means the compiler will be able to use floating point for some 
optimization. This will become a problem when receiving a trap as you 
would need to context switch the registers (this assume handler may use FP).
Cheers,

--
Julien Grall

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