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    SubjectRe: [RFC PATCH 1/5] KVM: vmx: speed up emulation of invalid guest state
    On Thu, Mar 27, 2014 at 12:30:34PM +0100, Paolo Bonzini wrote:
    > 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

    How about handling VM-entry error due to invalid state with

    vmx->emulation_required = true;
    continue to main vcpu loop;





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