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Commit b5c9ff73 authored by Takuya Yoshikawa's avatar Takuya Yoshikawa Committed by Avi Kivity
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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: default avatarTakuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Cc: Gleb Natapov <gleb@redhat.com>
Signed-off-by: default avatarAvi Kivity <avi@redhat.com>
parent adf52235

arch/x86/include/asm/kvm_emulate.h

+3 −2
Original line number Diff line number Diff line
@@ -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

+15 −2
Original line number Diff line number Diff line
@@ -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;

}