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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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?

-- 
Julien

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