From mboxrd@z Thu Jan 1 00:00:00 1970 From: Liran Alon Subject: Re: [PATCH] KVM: nVMX: Fix races when sending nested PI while dest enters/leaves L2 Date: Sat, 11 Nov 2017 00:37:32 +0200 Message-ID: <5A062A2C.9040102@ORACLE.COM> References: <1510252040-5609-1-git-send-email-liran.alon@oracle.com> <20171110180616.GE15561@flask> <0b202d55-6c87-e9b9-49aa-74aa7e83ff6b@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Cc: kvm@vger.kernel.org, jmattson@google.com, wanpeng.li@hotmail.com, idan.brown@ORACLE.COM, Krish Sadhukhan To: Paolo Bonzini , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= Return-path: Received: from userp1040.oracle.com ([156.151.31.81]:44075 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754604AbdKJWhm (ORCPT ); Fri, 10 Nov 2017 17:37:42 -0500 In-Reply-To: <0b202d55-6c87-e9b9-49aa-74aa7e83ff6b@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 10/11/17 23:37, Paolo Bonzini wrote: > On 10/11/2017 19:06, Radim Krčmář wrote: >>> /* the PIR and ON have been set by L1. */ >>> if (!kvm_vcpu_trigger_posted_interrupt(vcpu, true)) { >> This would still fail on the exiting case. >> >> If one VCPU was just after a VM exit, then the sender would see it >> IN_GUEST_MODE, send the posted notification and return true, but the >> notification would do nothing > > It would cause *something*---a vmexit because the vector doesn't match > the L1 posted interrupt. Then smp_kvm_posted_intr_nested_ipi would be > invoked from vmx_handle_external_intr. > > Could we detect the vector in vmx_handle_external_intr and set > pi_pending+KVM_REQ_EVENT? Or invoke a function in KVM from > smp_kvm_posted_intr_nested_ipi? Or would both be insane?... > I have actually thought about it before writing this patch. But have found an issue with this approach (which doesn't exist in this v1 patch and in Radim's suggestion for v2): Consider the case sender sees vcpu->mode==IN_GUEST_MODE and before it sends the physical IPI, dest CPU exits from guest and continues in L0 all the way until vcpu_enter_guest() and pass the part it checks for KVM_REQ_EVENT but before it disables interrupts. Then sender sends the physical IPI which is received in host-context and therefore runs smp_kvm_posted_intr_nested_ipi() which sets KVM_REQ_EVENT & pi_pending=true. But without Radim's suggestion of checking pi_pending after interrupts disabled, this is too late as dest CPU will not check these again until next exit from L2 guest. I hope I didn't misunderstand something here :) > Thanks, > > Paolo > >> and we didn't set vmx->nested.pi_pending = >> true, so the interrupt would not get handled until the next posted >> notification or nested VM entry. >> >