From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752923AbbIAEgN (ORCPT ); Tue, 1 Sep 2015 00:36:13 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56575 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750856AbbIAEgM (ORCPT ); Tue, 1 Sep 2015 00:36:12 -0400 Date: Tue, 1 Sep 2015 07:36:09 +0300 From: "Michael S. Tsirkin" To: Jason Wang Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Paolo Bonzini Subject: Re: [PATCH RFC 1/3] vmx: allow ioeventfd for EPT violations Message-ID: <20150901073305-mutt-send-email-mst@redhat.com> References: <1440925898-23440-1-git-send-email-mst@redhat.com> <1440925898-23440-2-git-send-email-mst@redhat.com> <55E51D69.2010303@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <55E51D69.2010303@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > --- > > 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.