On Wed, Sep 05, 2012 at 07:30:01PM +0900, Takuya Yoshikawa wrote:
> find_highest_vector() and count_vectors():
> - Instead of using magic values, define and use proper macros.
>
> find_highest_vector():
> - Remove likely() which is there only for historical reasons and not
> doing correct branch predictions anymore. Using such heuristics
> to optimize this function is not worth it now. Let CPUs predict
> things instead.
>
> - Stop checking word[0] separately. This was only needed for doing
> likely() optimization.
>
> - Use for loop, not while, to iterate over the register array to make
> the code clearer.
>
> Note that we actually confirmed that the likely() did wrong predictions
> by inserting debug code.
>
> Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx>
> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
Looks like a nice cleanup.
Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
> arch/x86/kvm/lapic.c | 30 ++++++++++++++++++------------
> 1 files changed, 18 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 18d149d..8ace252 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -66,6 +66,7 @@
> #define APIC_DEST_NOSHORT 0x0
> #define APIC_DEST_MASK 0x800
> #define MAX_APIC_VECTOR 256
> +#define APIC_VECTORS_PER_REG 32
>
> #define VEC_POS(v) ((v) & (32 - 1))
> #define REG_POS(v) (((v) >> 5) << 4)
> @@ -208,25 +209,30 @@ static unsigned int apic_lvt_mask[APIC_LVT_NUM] = {
>
> static int find_highest_vector(void *bitmap)
> {
> - u32 *word = bitmap;
> - int word_offset = MAX_APIC_VECTOR >> 5;
> + int vec;
> + u32 *reg;
>
> - while ((word_offset != 0) && (word[(--word_offset) << 2] == 0))
> - continue;
> + for (vec = MAX_APIC_VECTOR - APIC_VECTORS_PER_REG;
> + vec >= 0; vec -= APIC_VECTORS_PER_REG) {
> + reg = bitmap + REG_POS(vec);
> + if (*reg)
> + return fls(*reg) - 1 + vec;
> + }
>
> - if (likely(!word_offset && !word[0]))
> - return -1;
> - else
> - return fls(word[word_offset << 2]) - 1 + (word_offset << 5);
> + return -1;
> }
>
> static u8 count_vectors(void *bitmap)
> {
> - u32 *word = bitmap;
> - int word_offset;
> + int vec;
> + u32 *reg;
> u8 count = 0;
> - for (word_offset = 0; word_offset < MAX_APIC_VECTOR >> 5; ++word_offset)
> - count += hweight32(word[word_offset << 2]);
> +
> + for (vec = 0; vec < MAX_APIC_VECTOR; vec += APIC_VECTORS_PER_REG) {
> + reg = bitmap + REG_POS(vec);
> + count += hweight32(*reg);
> + }
> +
> return count;
> }
>
> --
> 1.7.5.4
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