Commit 74a497fa authored by Paolo Bonzini's avatar Paolo Bonzini Committed by Radim Krčmář
Browse files

KVM: nVMX: track dirty state of non-shadowed VMCS fields 



VMCS12 fields that are not handled through shadow VMCS are rarely
written, and thus they are also almost constant in the vmcs02.  We can
thus optimize prepare_vmcs02 by skipping all the work for non-shadowed
fields in the common case.

This patch introduces the (pretty simple) tracking infrastructure; the
next patches will move work to prepare_vmcs02_full and save a few hundred
clock cycles per VMRESUME on a Haswell Xeon E5 system:

	                                before  after
	cpuid                           14159   13869
	vmcall                          15290   14951
	inl_from_kernel                 17703   17447
	outl_to_kernel                  16011   14692
	self_ipi_sti_nop                16763   15825
	self_ipi_tpr_sti_nop            17341   15935
	wr_tsc_adjust_msr               14510   14264
	rd_tsc_adjust_msr               15018   14311
	mmio-wildcard-eventfd:pci-mem   16381   14947
	mmio-datamatch-eventfd:pci-mem  18620   17858
	portio-wildcard-eventfd:pci-io  15121   14769
	portio-datamatch-eventfd:pci-io 15761   14831

(average savings 748, stdev 460).

Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarRadim Krčmář <rkrcmar@redhat.com>
parent c9e9deae

arch/x86/kvm/vmx.c

+28 −1
Original line number Diff line number Diff line
@@ -436,6 +436,7 @@ struct nested_vmx { 
	 * data hold by vmcs12

	 */

	bool sync_shadow_vmcs;

	bool dirty_vmcs12;



	bool change_vmcs01_virtual_x2apic_mode;

	/* L2 must run next, and mustn't decide to exit to L1. */
@@ -7623,8 +7624,10 @@ static int handle_vmwrite(struct kvm_vcpu *vcpu) 
{

	unsigned long field;

	gva_t gva;

	struct vcpu_vmx *vmx = to_vmx(vcpu);

	unsigned long exit_qualification = vmcs_readl(EXIT_QUALIFICATION);

	u32 vmx_instruction_info = vmcs_read32(VMX_INSTRUCTION_INFO);



	/* The value to write might be 32 or 64 bits, depending on L1's long

	 * mode, and eventually we need to write that into a field of several

	 * possible lengths. The code below first zero-extends the value to 64
@@ -7667,6 +7670,20 @@ static int handle_vmwrite(struct kvm_vcpu *vcpu) 
		return kvm_skip_emulated_instruction(vcpu);

	}



	switch (field) {

#define SHADOW_FIELD_RW(x) case x:

#include "vmx_shadow_fields.h"

		/*

		 * The fields that can be updated by L1 without a vmexit are

		 * always updated in the vmcs02, the others go down the slow

		 * path of prepare_vmcs02.

		 */

		break;

	default:

		vmx->nested.dirty_vmcs12 = true;

		break;

	}



	nested_vmx_succeed(vcpu);

	return kvm_skip_emulated_instruction(vcpu);

}
@@ -7681,6 +7698,7 @@ static void set_current_vmptr(struct vcpu_vmx *vmx, gpa_t vmptr) 
			     __pa(vmx->vmcs01.shadow_vmcs));

		vmx->nested.sync_shadow_vmcs = true;

	}

	vmx->nested.dirty_vmcs12 = true;

}



/* Emulate the VMPTRLD instruction */
@@ -10297,6 +10315,11 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool ne 
	return 0;

}



static void prepare_vmcs02_full(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,

			       bool from_vmentry)

{

}



/*

 * prepare_vmcs02 is called when the L1 guest hypervisor runs its nested

 * L2 guest. L1 has a vmcs for L2 (vmcs12), and this function "merges" it
@@ -10592,7 +10615,6 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12, 
			vmcs_write16(VIRTUAL_PROCESSOR_ID, vmx->vpid);

			vmx_flush_tlb(vcpu, true);

		}



	}



	if (enable_pml) {
@@ -10641,6 +10663,11 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12, 
	/* Note: modifies VM_ENTRY/EXIT_CONTROLS and GUEST/HOST_IA32_EFER */

	vmx_set_efer(vcpu, vcpu->arch.efer);



	if (vmx->nested.dirty_vmcs12) {

		prepare_vmcs02_full(vcpu, vmcs12, from_vmentry);

		vmx->nested.dirty_vmcs12 = false;

	}



	/* Shadow page tables on either EPT or shadow page tables. */

	if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3, nested_cpu_has_ept(vmcs12),

				entry_failure_code))

arch/x86/kvm/vmx_shadow_fields.h

+6 −0
Original line number Diff line number Diff line
@@ -17,6 +17,12 @@ 
 * (e.g. force a sync if VM_INSTRUCTION_ERROR is modified

 * by nested_vmx_failValid)

 *

 * When adding or removing fields here, note that shadowed

 * fields must always be synced by prepare_vmcs02, not just

 * prepare_vmcs02_full.

 */



/*

 * Keeping the fields ordered by size is an attempt at improving

 * branch prediction in vmcs_read_any and vmcs_write_any.

 */

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