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>

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;

arch/x86/kvm/x86.c

@@ -4506,6 +4506,20 @@ static void inject_emulated_exception(struct kvm_vcpu *vcpu)
 		kvm_queue_exception(vcpu, ctxt->exception.vector);
 }
 
+static void init_decode_cache(struct decode_cache *c,
+			      const unsigned long *regs)
+{
+	memset(c, 0, offsetof(struct decode_cache, regs));
+	memcpy(c->regs, regs, sizeof(c->regs));
+
+	c->fetch.start = 0;
+	c->fetch.end = 0;
+	c->io_read.pos = 0;
+	c->io_read.end = 0;
+	c->mem_read.pos = 0;
+	c->mem_read.end = 0;
+}
+
 static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
 {
 	struct x86_emulate_ctxt *ctxt = &vcpu->arch.emulate_ctxt;
@@ -4531,8 +4545,7 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
 							  X86EMUL_MODE_PROT16;
 	ctxt->guest_mode = is_guest_mode(vcpu);
 
-	memset(c, 0, sizeof(struct decode_cache));
-	memcpy(c->regs, vcpu->arch.regs, sizeof c->regs);
+	init_decode_cache(c, vcpu->arch.regs);
 	vcpu->arch.emulate_regs_need_sync_from_vcpu = false;
 }