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

[Peter Maydell <peter.maydell@linaro.org>]

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

thanks
-- PMM