Re: [PATCH 2/3] KVM: VMX: Short circuit STI; HLT while an interrupt is pending

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Feb 16, 2011 at 11:01:47AM +0200, Avi Kivity wrote:
> On 02/15/2011 10:36 PM, Marcelo Tosatti wrote:
> >On Mon, Feb 14, 2011 at 04:42:16PM +0200, Avi Kivity wrote:
> >>  Short-circuit an STI; HLT sequence while an interrupt is pending:
> >>  instead of halting, re-entering the guest, and exiting immediately
> >>  on an interrupt window exit, go directly to the last step.
> >>
> >>  Saves a vmexit on workloads where interrupts are received synchronously;
> >>  an example is a disk backed by the host page cache where there is no
> >>  latency (from the guest's point of view) between the request and fulfilment.
> >>
> >>  Signed-off-by: Avi Kivity<avi@redhat.com>
> >>  ---
> >>   arch/x86/kvm/vmx.c |    9 +++++++++
> >>   1 files changed, 9 insertions(+), 0 deletions(-)
> >>
> >>  diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> >>  index ee1cd1a..541da0e 100644
> >>  --- a/arch/x86/kvm/vmx.c
> >>  +++ b/arch/x86/kvm/vmx.c
> >>  @@ -3437,6 +3437,15 @@ static int handle_interrupt_window(struct kvm_vcpu *vcpu)
> >>   static int handle_halt(struct kvm_vcpu *vcpu)
> >>   {
> >>   	skip_emulated_instruction(vcpu);
> >>  +	/*
> >>  +	 * Short-circuit an STI; HLT sequence while an interrupt is pending:
> >>  +	 * instead of halting, re-entering the guest, and exiting immediately
> >>  +	 * on an interrupt window exit, go directly to the last step.
> >>  +	 */
> >>  +	if ((to_vmx(vcpu)->cpu_based_vm_exec_control
> >>  +	&  CPU_BASED_VIRTUAL_INTR_PENDING)
> >>  +	&&  (kvm_get_rflags(vcpu)&  X86_EFLAGS_IF))
> >>  +		return handle_interrupt_window(vcpu);
> >>   	return kvm_emulate_halt(vcpu);
> >>   }
> >
> >Why does the normal vcpu entry path fails to inject the interrupt? Because after halt,
> >KVM_REQ_EVENT is not set?
> 
> Yes.  Also, we want to clear CPU_BASED_VIRTUAL_INTR_PENDING.

Is there a reason why it cannot be handled in the main loop?