KVM: x86 emulator: Avoid clearing the whole decode_cache

During tracing the emulator, we noticed that init_emulate_ctxt()
sometimes took a bit longer time than we expected.

This patch is for mitigating the problem by some degree.

By looking into the function, we soon notice that it clears the whole
decode_cache whose size is about 2.5K bytes now.  Furthermore, most of
the bytes are taken for the two read_cache arrays, which are used only
by a few instructions.

Considering the fact that we are not assuming the cache arrays have
been cleared when we store actual data, we do not need to clear the
arrays: 2K bytes elimination.  In addition, we can avoid clearing the
fetch_cache and regs arrays.

This patch changes the initialization not to clear the arrays.

On our 64-bit host, init_emulate_ctxt() becomes 0.3 to 0.5us faster with
this patch applied.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Cc: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
index ab09ba290db3..c0f77e09ebce 100644
--- a/arch/x86/include/asm/kvm_emulate.h
+++ b/arch/x86/include/asm/kvm_emulate.h
@@ -246,8 +246,6 @@ struct decode_cache {
 	unsigned int d;
 	int (*execute)(struct x86_emulate_ctxt *ctxt);
 	int (*check_perm)(struct x86_emulate_ctxt *ctxt);
-	unsigned long regs[NR_VCPU_REGS];
-	unsigned long eip;
 	/* modrm */
 	u8 modrm;
 	u8 modrm_mod;
@@ -255,6 +253,9 @@ struct decode_cache {
 	u8 modrm_rm;
 	u8 modrm_seg;
 	bool rip_relative;
+	unsigned long eip;
+	/* Fields above regs are cleared together. */
+	unsigned long regs[NR_VCPU_REGS];
 	struct fetch_cache fetch;
 	struct read_cache io_read;
 	struct read_cache mem_read;