Re: [Minios-devel] [UNIKRAFT PATCH v3 3/7] plat/kvm: arm64: Enable the fp/simd at the starting point

[Justin He <Justin.He@xxxxxxx>]

Hi Sharan

> -----Original Message-----
> From: Sharan Santhanam <sharan.santhanam@xxxxxxxxx>
> Sent: Monday, April 6, 2020 5:38 PM
> To: Justin He <Justin.He@xxxxxxx>; minios-devel@xxxxxxxxxxxxxxxxxxxx;
> Simon Kuenzer <simon.kuenzer@xxxxxxxxx>; Felipe Huici
> <felipe.huici@xxxxxxxxx>
> Cc: Kaly Xin <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> Subject: Re: [UNIKRAFT PATCH v3 3/7] plat/kvm: arm64: Enable the fp/simd
> at the starting point
> 
> 
> On 4/2/20 6:18 AM, 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.
> >
> > 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
> 
> Do we need to enable the floating point upon entry?
> 
> It might be better to do it at the setup phase in `setup.c` and
> implement this as inline assembly
If we don't enable fpsimd upon entry, we need to carefully NOT invoking
any uk_pr_*/printf debugging interface before enabling fpsimd. Otherwise
the guest will be trapped into floatingpoint exception and hang forever. 
Please note that we can't prevent the uk_pr_* or printf from using q0-q31.

What do you think of it?

--
Cheers,
Justin (Jia He)

 
> Since it is a cpu feature, it might be better to provide functions to
> enable and disable it.
> 
> > +
> >     /* Calculate the image size */
> >     ldr x25, =_dtb
> >     ldr x26, =_end
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