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>
---
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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