Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751882AbdHPOKo (ORCPT ); Wed, 16 Aug 2017 10:10:44 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53370 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751370AbdHPOKm (ORCPT ); Wed, 16 Aug 2017 10:10:42 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com C64299B42A Authentication-Results: ext-mx10.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx10.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=mst@redhat.com Date: Wed, 16 Aug 2017 17:10:41 +0300 From: "Michael S. Tsirkin" To: Paolo Bonzini Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, rkrcmar@redhat.com, stable@vger.kernel.org, Jason Wang Subject: Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO Message-ID: <20170816170843-mutt-send-email-mst@kernel.org> References: <1502890494-35208-1-git-send-email-pbonzini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1502890494-35208-1-git-send-email-pbonzini@redhat.com> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Wed, 16 Aug 2017 14:10:42 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3267 Lines: 75 On Wed, Aug 16, 2017 at 03:34:54PM +0200, Paolo Bonzini wrote: > Microsoft pointed out privately to me that KVM's handling of > KVM_FAST_MMIO_BUS is invalid. Using skip_emulation_instruction is invalid > in EPT misconfiguration vmexit handlers, because neither EPT violations > nor misconfigurations are listed in the manual among the VM exits that > set the VM-exit instruction length field. > > While physical processors seem to set the field, this is not architectural > and is just a side effect of the implementation. I couldn't convince > myself of any condition on the exit qualification where VM-exit > instruction length "has" to be defined; there are no trap-like VM-exits > that can be repurposed; and fault-like VM-exits such as descriptor-table > exits provide no decoding information. So I don't really see any way > to keep the full speedup. > > What we can do is use EMULTYPE_SKIP; it only saves 200 clock cycles > because computing the physical RIP and reading the instruction is > expensive, but at least the eventfd is signaled before entering the > emulator. This saves on latency. While at it, don't check breakpoints > when skipping the instruction, as presumably any side effect has been > exposed already. > > Adding a hypercall or MSR write that does a fast MMIO write to a physical > address would do it, but it adds hypervisor knowledge in virtio, including > CPUID handling. So it would be pretty ugly in the guest-side implementation, > but if somebody wants to do it and the virtio side is acceptable to the > virtio maintainers, I am okay with it. > > Cc: Michael S. Tsirkin > Cc: stable@vger.kernel.org > Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae > Suggested-by: Radim Krčmář > Signed-off-by: Paolo Bonzini Jason (cc) who worked on the original optimization said he can work to test the performance impact. I suggest we don't rush this (it's been like this for 2 years), and the issue seems to be largely theoretical. > --- > arch/x86/kvm/vmx.c | 3 ++- > arch/x86/kvm/x86.c | 3 ++- > 2 files changed, 4 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > index df8d2f127508..5ec47fd0b990 100644 > --- a/arch/x86/kvm/vmx.c > +++ b/arch/x86/kvm/vmx.c > @@ -6407,7 +6407,8 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu) > gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS); > if (!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) { > trace_kvm_fast_mmio(gpa); > - return kvm_skip_emulated_instruction(vcpu); > + return x86_emulate_instruction(vcpu, gpa, EMULTYPE_SKIP, > + NULL, 0) == EMULATE_DONE; > } > > ret = handle_mmio_page_fault(vcpu, gpa, true); > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index e10eda86bc7b..e74b79dab343 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -5654,7 +5654,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, > * handle watchpoints yet, those would be handled in > * the emulate_ops. > */ > - if (kvm_vcpu_check_breakpoint(vcpu, &r)) > + if (!(emulation_type & EMULTYPE_SKIP) && > + kvm_vcpu_check_breakpoint(vcpu, &r)) > return r; > > ctxt->interruptibility = 0; > -- > 1.8.3.1