Re: [PATCH RFC 1/3] vmx: allow ioeventfd for EPT violations

[Jason Wang <jasowang@redhat.com>]

On 09/01/2015 12:36 PM, Michael S. Tsirkin wrote:
> On Tue, Sep 01, 2015 at 11:37:13AM +0800, Jason Wang wrote:
>> > 
>> > 
>> > On 08/30/2015 05:12 PM, Michael S. Tsirkin wrote:
>>> > > Even when we skip data decoding, MMIO is slightly slower
>>> > > than port IO because it uses the page-tables, so the CPU
>>> > > must do a pagewalk on each access.
>>> > >
>>> > > This overhead is normally masked by using the TLB cache:
>>> > > but not so for KVM MMIO, where PTEs are marked as reserved
>>> > > and so are never cached.
>>> > >
>>> > > As ioeventfd memory is never read, make it possible to use
>>> > > RO pages on the host for ioeventfds, instead.
>>> > > The result is that TLBs are cached, which finally makes MMIO
>>> > > as fast as port IO.
>>> > >
>>> > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>>> > > ---
>>> > >  arch/x86/kvm/vmx.c | 5 +++++
>>> > >  1 file changed, 5 insertions(+)
>>> > >
>>> > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> > > index 9d1bfd3..ed44026 100644
>>> > > --- a/arch/x86/kvm/vmx.c
>>> > > +++ b/arch/x86/kvm/vmx.c
>>> > > @@ -5745,6 +5745,11 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
>>> > >  		vmcs_set_bits(GUEST_INTERRUPTIBILITY_INFO, GUEST_INTR_STATE_NMI);
>>> > >  
>>> > >  	gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
>>> > > +	if (!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
>>> > > +		skip_emulated_instruction(vcpu);
>>> > > +		return 1;
>>> > > +	}
>>> > > +
>>> > >  	trace_kvm_page_fault(gpa, exit_qualification);
>>> > >  
>>> > >  	/* It is a write fault? */
>> > 
>> > Just notice that vcpu_mmio_write() tries lapic first. Should we do the
>> > same here? Otherwise we may slow down apic access consider we may have
>> > hundreds of eventfds.
> IIUC this does not affect mmio at all: for mmio we set
> reserved page flag, so they trigger an EPT misconfiguration,
> not an EPT violation.

I see, so the question could be asked for current misconfiguration
handler instead?