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Subject[PATCH 2/4] KVM: nVMX: track dirty state of non-shadowed VMCS fields
Date
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: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/vmx.c | 29 ++++++++++++++++++++++++++++-
1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 2ee842990976..8b6013b529b3 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -441,6 +441,7 @@ 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. */
@@ -7879,8 +7880,10 @@ 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
@@ -7923,6 +7926,20 @@ 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);
}
@@ -7937,6 +7954,7 @@ 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 */
@@ -10569,6 +10587,11 @@ 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
@@ -10864,7 +10887,6 @@ 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) {
@@ -10913,6 +10935,11 @@ 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))
--
1.8.3.1

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