KVM: x86 emulator: Avoid clearing the whole decode_cache (b5c9ff73) · Commits · 戴 / test

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

+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;
		}

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