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

[Justin He <Justin.He@xxxxxxx>]

Hi Julien

> -----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.
Hence, if we want to disable fp/simd in unikraft kernel and enable it in
userspace, we need to specify different compilation options with/without
-mgeneral-regs-only for each unkraft kernel library. I don't know whether
it is acceptable? @Simon Kuenzer What do you think of it?

--
Cheers,
Justin (Jia He)


> >
> > Signed-off-by: Jia He <justin.he@xxxxxxx>
> > ---
> >   plat/kvm/arm/entry64.S | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> >
> > diff --git a/plat/kvm/arm/entry64.S b/plat/kvm/arm/entry64.S
> > index 359a310..1e32268 100644
> > --- a/plat/kvm/arm/entry64.S
> > +++ b/plat/kvm/arm/entry64.S
> > @@ -36,6 +36,7 @@
> >   #include <kvm-arm/mm.h>
> >   #include <arm/cpu_defs.h>
> >   #include <uk/plat/common/sections.h>
> > +#include <uk/config.h>
> >
> >   .global page_table_size
> >   .data
> > @@ -49,6 +50,13 @@ page_table_size:
> >
> >   .text
> >   ENTRY(_libkvmplat_entry)
> > +#ifdef CONFIG_FLOAT_POINT
> > +   /* Enable fp/simd support */
> > +   ldr        x0, =(3 << 20)
> > +   msr        cpacr_el1, x0
> > +   isb
> > +#endif
> > +
> >     /* Calculate the image size */
> >     ldr x25, =_dtb
> >     ldr x26, =_end
> >
>
> --
> Julien Grall
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