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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2013-06-03 at 15:32 +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


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