KVM: x86: lapic: Clean up find_highest_vector() and count_vectors() (ecba9a52) · Commits · 戴 / test

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. Acked-by:

Michael S. Tsirkin <mst@redhat.com> Signed-off-by:

Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp> Signed-off-by:

Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/lapic.c

+18 −12

Original line number	Diff line number	Diff line
		@@ -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 const 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);
		}

		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;
		}

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