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Commit b2da15ac authored by Avi Kivity's avatar Avi Kivity Committed by Marcelo Tosatti
Browse files

KVM: VMX: Optimize %ds, %es reload 



On x86_64, we can defer %ds and %es reload to the heavyweight context switch,
since nothing in the lightweight paths uses the host %ds or %es (they are
ignored by the processor).  Furthermore we can avoid the load if the segments
are null, by letting the hardware load the null segments for us.  This is the
expected case.

On i386, we could avoid the reload entirely, since the entry.S paths take care
of reload, except for the SYSEXIT path which leaves %ds and %es set to __USER_DS.
So we set them to the same values as well.

Saves about 70 cycles out of 1600 (around 4%; noisy measurements).

Signed-off-by: default avatarAvi Kivity <avi@redhat.com>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent 512d5649

arch/x86/kvm/vmx.c

+31 −5
Original line number Diff line number Diff line
@@ -393,6 +393,9 @@ struct vcpu_vmx { 
	struct {

		int           loaded;

		u16           fs_sel, gs_sel, ldt_sel;

#ifdef CONFIG_X86_64

		u16           ds_sel, es_sel;

#endif

		int           gs_ldt_reload_needed;

		int           fs_reload_needed;

	} host_state;
@@ -1417,6 +1420,11 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu) 
		vmx->host_state.gs_ldt_reload_needed = 1;

	}



#ifdef CONFIG_X86_64

	savesegment(ds, vmx->host_state.ds_sel);

	savesegment(es, vmx->host_state.es_sel);

#endif



#ifdef CONFIG_X86_64

	vmcs_writel(HOST_FS_BASE, read_msr(MSR_FS_BASE));

	vmcs_writel(HOST_GS_BASE, read_msr(MSR_GS_BASE));
@@ -1457,6 +1465,19 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx) 
	}

	if (vmx->host_state.fs_reload_needed)

		loadsegment(fs, vmx->host_state.fs_sel);

#ifdef CONFIG_X86_64

	if (unlikely(vmx->host_state.ds_sel | vmx->host_state.es_sel)) {

		loadsegment(ds, vmx->host_state.ds_sel);

		loadsegment(es, vmx->host_state.es_sel);

	}

#else

	/*

	 * The sysexit path does not restore ds/es, so we must set them to

	 * a reasonable value ourselves.

	 */

	loadsegment(ds, __USER_DS);

	loadsegment(es, __USER_DS);

#endif

	reload_tss();

#ifdef CONFIG_X86_64

	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
@@ -3640,8 +3661,18 @@ static void vmx_set_constant_host_state(void) 
	vmcs_writel(HOST_CR3, read_cr3());  /* 22.2.3  FIXME: shadow tables */



	vmcs_write16(HOST_CS_SELECTOR, __KERNEL_CS);  /* 22.2.4 */

#ifdef CONFIG_X86_64

	/*

	 * Load null selectors, so we can avoid reloading them in

	 * __vmx_load_host_state(), in case userspace uses the null selectors

	 * too (the expected case).

	 */

	vmcs_write16(HOST_DS_SELECTOR, 0);

	vmcs_write16(HOST_ES_SELECTOR, 0);

#else

	vmcs_write16(HOST_DS_SELECTOR, __KERNEL_DS);  /* 22.2.4 */

	vmcs_write16(HOST_ES_SELECTOR, __KERNEL_DS);  /* 22.2.4 */

#endif

	vmcs_write16(HOST_SS_SELECTOR, __KERNEL_DS);  /* 22.2.4 */

	vmcs_write16(HOST_TR_SELECTOR, GDT_ENTRY_TSS*8);  /* 22.2.4 */
@@ -6102,10 +6133,7 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx) 
static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)

{

	struct vcpu_vmx *vmx = to_vmx(vcpu);

	u16 _ds, _es;



	savesegment(ds, _ds);

	savesegment(es, _es);

	if (is_guest_mode(vcpu) && !vmx->nested.nested_run_pending) {

		struct vmcs12 *vmcs12 = get_vmcs12(vcpu);

		if (vmcs12->idt_vectoring_info_field &
@@ -6266,8 +6294,6 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu) 
		}

	}



	loadsegment(ds, _ds);

	loadsegment(es, _es);

	vmx->loaded_vmcs->launched = 1;



	vmx->exit_reason = vmcs_read32(VM_EXIT_REASON);