Re: [Xen-devel] [PATCH v3 2/2] xen/arm32: implement VFP context switch

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2013-06-03 at 15:57 +0100, Julien Grall wrote:
> On 06/03/2013 03:38 PM, Ian Campbell wrote:
> 
> > On Mon, 2013-06-03 at 15:32 +0100, Julien Grall wrote:
> >> On 06/03/2013 03:15 PM, Ian Campbell wrote:
> >>
> >>> On Mon, 2013-06-03 at 15:00 +0100, Julien Grall wrote:
> >>>> Add support for VFP context switch on arm32 and a dummy support for arm64
> >>>>
> >>>> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> >>>>
> >>>> Changes in v3:
> >>>>     - Add vfp_init to check if the processor supports VFP 3
> >>>>     - Add clobber memory
> >>>>     - Remove tmps
> >>>>     - s/COFNIG_ARM64/CONFIG_ARM64/ in include/asm/arm.h
> >>>>
> >>>> Changes in v2:
> >>>>     - Fix all the small errors (type, lost headers...)
> >>>>     - Add some comments
> >>>> ---
> >>>>  xen/arch/arm/arm32/Makefile     |    1 +
> >>>>  xen/arch/arm/arm32/vfp.c        |   99 
> >>>> +++++++++++++++++++++++++++++++++++++++
> >>>>  xen/arch/arm/arm64/Makefile     |    1 +
> >>>>  xen/arch/arm/arm64/vfp.c        |   13 +++++
> >>>>  xen/arch/arm/domain.c           |    7 ++-
> >>>>  xen/include/asm-arm/arm32/vfp.h |   41 ++++++++++++++++
> >>>>  xen/include/asm-arm/arm64/vfp.h |   16 +++++++
> >>>>  xen/include/asm-arm/cpregs.h    |    9 ++++
> >>>>  xen/include/asm-arm/domain.h    |    4 ++
> >>>>  xen/include/asm-arm/vfp.h       |   25 ++++++++++
> >>>>  10 files changed, 214 insertions(+), 2 deletions(-)
> >>>>  create mode 100644 xen/arch/arm/arm32/vfp.c
> >>>>  create mode 100644 xen/arch/arm/arm64/vfp.c
> >>>>  create mode 100644 xen/include/asm-arm/arm32/vfp.h
> >>>>  create mode 100644 xen/include/asm-arm/arm64/vfp.h
> >>>>  create mode 100644 xen/include/asm-arm/vfp.h
> >>>>
> >>>> diff --git a/xen/arch/arm/arm32/Makefile b/xen/arch/arm/arm32/Makefile
> >>>> index aaf277a..b903803 100644
> >>>> --- a/xen/arch/arm/arm32/Makefile
> >>>> +++ b/xen/arch/arm/arm32/Makefile
> >>>> @@ -6,5 +6,6 @@ obj-y += proc-ca15.o
> >>>>  
> >>>>  obj-y += traps.o
> >>>>  obj-y += domain.o
> >>>> +obj-y += vfp.o
> >>>>  
> >>>>  obj-$(EARLY_PRINTK) += debug.o
> >>>> diff --git a/xen/arch/arm/arm32/vfp.c b/xen/arch/arm/arm32/vfp.c
> >>>> new file mode 100644
> >>>> index 0000000..2ece43d
> >>>> --- /dev/null
> >>>> +++ b/xen/arch/arm/arm32/vfp.c
> >>>> @@ -0,0 +1,99 @@
> >>>> +#include <xen/sched.h>
> >>>> +#include <xen/init.h>
> >>>> +#include <asm/processor.h>
> >>>> +#include <asm/vfp.h>
> >>>> +
> >>>> +void vfp_save_state(struct vcpu *v)
> >>>> +{
> >>>> +    v->arch.vfp.fpexc = READ_CP32(FPEXC);
> >>>> +
> >>>> +    WRITE_CP32(v->arch.vfp.fpexc | FPEXC_EN, FPEXC);
> >>>> +
> >>>> +    v->arch.vfp.fpscr = READ_CP32(FPSCR);
> >>>> +
> >>>> +    if ( v->arch.vfp.fpexc & FPEXC_EX ) /* Check for sub-architecture */
> >>>> +    {
> >>>> +        v->arch.vfp.fpinst = READ_CP32(FPINST);
> >>>> +
> >>>> +        if ( v->arch.vfp.fpexc & FPEXC_FP2V )
> >>>> +            v->arch.vfp.fpinst2 = READ_CP32(FPINST2);
> >>>> +        /* Disable FPEXC_EX */
> >>>> +        WRITE_CP32((v->arch.vfp.fpexc | FPEXC_EN) & ~FPEXC_EX, FPEXC);
> >>>> +    }
> >>>> +
> >>>> +    /* Save {d0-d15} */
> >>>> +    asm volatile("stc p11, cr0, [%0], #32*4"
> >>>> +                 : : "r" (v->arch.vfp.fpregs1)
> >>>> +                 : "memory");
> >>>
> >>> http://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html suggests that
> >>> "=m" (v->arch.vfp.fpregs1) or "=Q" (...) as output constraints might do
> >>> the job without clobbering the whole of memory.
> >>
> >> I'm not sure to fully understand the concept behind "=m". Does it mean
> >> that gcc will clobber all the memory range addressed by fpregs?
> > 
> > I'm not totally confident in this stuff myself....
> > 
> > Apparently the "=" modified means[0] "this operand is write-only for
> > this instruction: the previous value is discarded and replaced by output
> > data." In the case of a memory constraint you'd want to hope that
> > "discarded and replaced" would be equivalent to clobbering the address,
> > at least in cases where the compiler knows the size of the thing, as it
> > does here.
> > 
> > Ian.
> > 
> > [0] http://gcc.gnu.org/onlinedocs/gcc-4.8.1/gcc/Modifiers.html#Modifiers
> > 
> 
> 
> Shall I use this contrainst for the stc instructions and send a new
> patch series?

If you buy my reasoning and if it works then I guess so.

Ian.



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