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

[Julien Grall <julien@xxxxxxx>]

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.

> 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?
See above, if you decide to enable FPSIMD for your kernel then this 
series is not enough.
However, this is likely a waste of time as the kernel itself should have 
limited use of FPSIMD. Now imagine for SVE....
So I would highly recommend to ensure the kernel (particularly the 
exception path) is free of FPSIMD.

> --
> 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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Cheers,

[1] 
https://lists.xenproject.org/archives/html/minios-devel/2018-07/msg00038.html
--
Julien Grall

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