Re: [Qemu-devel] [PULL for-2.0 2/2] cpu: Avoid QOM casts for CPU()

[Laurent Desnogues]

On Mon, Mar 31, 2014 at 7:37 PM, Andreas Färber <address@hidden> 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 to occur elsewhere
> anyway.
>
> Keep the CPU() macro for a consistent developer experience and for
> flexibility to exchange its implementation, but turn it into a pure,
> unchecked C cast for now.

I re-ran my image to measure the improvements.

Intel 4770K (no HT, no OC)
Fedora Core 19
QEMU 82c6f513735297ad76acaaf2e87f0c5a0b3647a7
Image:  ARM kernel + Google V8 running Sunspider

old: without this patch
strong: default stack protection (strong in my case)
noprot: no stack protection (--disable-stack-protector)
disable: no QOM debug (--disable-qom-cast-debug)

old+strong         16.7
old+strong+disable 15.9
old+noprot         16.6
old+noprot+disable 16.0
    strong         15.8
    strong+disable 15.7
    noprot         15.6
    noprot+disable 15.5

Thanks,

Laurent

> Compare commit 6e42be7cd10260fd3a006d94f6c870692bf7a2c0.
>
> Reported-by: Laurent Desnogues <address@hidden>
> Suggested-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Andreas Färber <address@hidden>
> ---
>  include/qom/cpu.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index f99885a..df977c8 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -53,7 +53,12 @@ typedef uint64_t vaddr;
>
>  #define TYPE_CPU "cpu"
>
> -#define CPU(obj) OBJECT_CHECK(CPUState, (obj), TYPE_CPU)
> +/* Since this macro is used a lot in hot code paths and in conjunction with
> + * FooCPU *foo_env_get_cpu(), we deviate from usual QOM practice 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
>
>