From mboxrd@z Thu Jan 1 00:00:00 1970 From: Radim =?utf-8?B?S3LEjW3DocWZ?= Subject: Re: [PATCH] KVM: nVMX: Fix races when sending nested PI while dest enters/leaves L2 Date: Thu, 16 Nov 2017 18:37:15 +0100 Message-ID: <20171116173714.GC20438@flask> References: <1510252040-5609-1-git-send-email-liran.alon@oracle.com> <20171110180616.GE15561@flask> <0b202d55-6c87-e9b9-49aa-74aa7e83ff6b@redhat.com> <5A062A2C.9040102@ORACLE.COM> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: Paolo Bonzini , kvm@vger.kernel.org, jmattson@google.com, wanpeng.li@hotmail.com, idan.brown@ORACLE.COM, Krish Sadhukhan To: Liran Alon Return-path: Received: from mx1.redhat.com ([209.132.183.28]:7958 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933322AbdKPRhT (ORCPT ); Thu, 16 Nov 2017 12:37:19 -0500 Content-Disposition: inline In-Reply-To: <5A062A2C.9040102@ORACLE.COM> Sender: kvm-owner@vger.kernel.org List-ID: 2017-11-11 00:37+0200, Liran Alon: > 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 :) kvm_request_pending() would notice KVM_REQ_EVENT and forces the VM entry to restart, so that wouldn't be a problem. I just realized another complication, though: when the sender looks at IN_GUEST_MODE and before it sends IPI, the destination can reschedule to a different VCPU => the IPI handler cannot use the 'current VCPU' and we have to build a list of VCPUs potentially awaiting notification vector for every PCPU, which makes it strictly worse than just looking directly at the ON bit, IMO.