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Subject[RFC PATCH 1/5] KVM: vmx: speed up emulation of invalid guest state
Date
About 25% of the time spent in emulation of invalid guest state
is wasted in checking whether emulation is required for the next
instruction. However, this almost never changes except when a
segment register (or TR or LDTR) changes, or when there is a mode
transition (i.e. CR0 changes).

In fact, vmx_set_segment and vmx_set_cr0 already modify
vmx->emulation_required (except that the former for some reason
uses |= instead of just an assignment). So there is no need to
call guest_state_valid in the emulation loop.

Emulation performance test results indicate 530-870 cycles
for common instructions, versus 740-1110 before this patch.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/vmx.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 1320e0f8e611..73aa522db47b 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3629,7 +3629,7 @@ static void vmx_set_segment(struct kvm_vcpu *vcpu,
vmcs_write32(sf->ar_bytes, vmx_segment_access_rights(var));

out:
- vmx->emulation_required |= emulation_required(vcpu);
+ vmx->emulation_required = emulation_required(vcpu);
}

static void vmx_get_cs_db_l_bits(struct kvm_vcpu *vcpu, int *db, int *l)
@@ -5580,7 +5580,7 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
cpu_exec_ctrl = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
intr_window_requested = cpu_exec_ctrl & CPU_BASED_VIRTUAL_INTR_PENDING;

- while (!guest_state_valid(vcpu) && count-- != 0) {
+ while (vmx->emulation_required && count-- != 0) {
if (intr_window_requested && vmx_interrupt_allowed(vcpu))
return handle_interrupt_window(&vmx->vcpu);

@@ -5614,7 +5614,6 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
schedule();
}

- vmx->emulation_required = emulation_required(vcpu);
out:
return ret;
}
--
1.8.3.1



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