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

[Santiago Pagani <Santiago.Pagani@xxxxxxxxx>]

Hi Justin,

Actually, the comment is wrong. Setting those two bits in cpacr_el1 does not 
enable fp/simd, but actually disables traps for floating point and SIMD 
instructions both in EL0 and EL1.
The actually enabling of floating point and SIMD instructions is done by your 
other patch that removes the -mgeneral-regs-only flag.
Therefore, there is no harm in doing that in the kernel when floating point is 
enabled. I would however fix the comment though.

Thanks and best,
Santiago

On 20.12.19, 02:14, "Justin He" <Justin.He@xxxxxxx> wrote:

    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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