Re: [Qemu-devel] [PATCH qom-cpu for-2.0] cpu: Avoid QOM casts for CPU()

["Andreas Färber" <afaerber@suse.de>]

Am 28.03.2014 16:52, schrieb Peter Maydell:
> On 28 March 2014 15:49, Andreas Färber <afaerber@suse.de> wrote:
>> CPU address spaces touching load and store helpers as well as the
>> movement of (almost) all fields from CPU_COMMON to CPUState have led to
>> a noticeable increase of CPU() usage in "hot" paths for both TCG and KVM.
>>
>> While CPU()'s OBJECT_CHECK() might help detect development errors, i.e. in
>> form of crashes due to QOM vs. non-QOM mismatches rather than QOM type
>> mismatches, it is not really needed at runtime since mostly used in
>> CPU-specific paths, coming from a target-specific CPU subtype. If that
>> pointer is damaged, other errors are highly likely occur elsewhere anyway.

"to occur"

>>
>> Keep the CPU() macro for a consistent developer experience and

"and for"

>> flexibility to exchange its implementation, but turn it into a pure
>> C cast for now.
>>
>> Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
>> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
>> Signed-off-by: Andreas Färber <afaerber@suse.de>
>> ---
>>  include/qom/cpu.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
>> index f99885a..0aa1bdc 100644
>> --- a/include/qom/cpu.h
>> +++ b/include/qom/cpu.h
>> @@ -53,7 +53,7 @@ typedef uint64_t vaddr;
>>
>>  #define TYPE_CPU "cpu"
>>
>> -#define CPU(obj) OBJECT_CHECK(CPUState, (obj), TYPE_CPU)

/* Avoid unnecessary penalties in hot paths by using an unchecked cast. */ ?

>> +#define CPU(obj) ((CPUState *)(obj))
>>  #define CPU_CLASS(class) OBJECT_CLASS_CHECK(CPUClass, (class), TYPE_CPU)
>>  #define CPU_GET_CLASS(obj) OBJECT_GET_CLASS(CPUClass, (obj), TYPE_CPU)
>>
>> --
>> 1.8.4.5
> 
> Not important if we're going to revert this as soon as 2.0
> is out of the door, but if we expect this to be around for
> longer than that it would be worth a brief comment explaining
> why this cast macro is a special case.

Right now I don't see a post-2.0 solution emerging, so yes,
documentation is good.

Cheers,
Andreas

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