From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754720AbdCINLF (ORCPT ); Thu, 9 Mar 2017 08:11:05 -0500 Received: from mx1.redhat.com ([209.132.183.28]:55650 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750770AbdCINLD (ORCPT ); Thu, 9 Mar 2017 08:11:03 -0500 Subject: Re: [PATCH] KVM: x86: optimize local IRQ disable time before vmentry To: Paolo Bonzini , Wanpeng Li , linux-kernel@vger.kernel.org, kvm@vger.kernel.org References: <1489061763-3832-1-git-send-email-wanpeng.li@hotmail.com> <7939e90b-4fe0-d07a-82b4-73cb0f4407fc@redhat.com> Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Wanpeng Li From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <994702e9-1d62-0ac4-3845-8bc9e816c284@redhat.com> Date: Thu, 9 Mar 2017 14:11:00 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0 MIME-Version: 1.0 In-Reply-To: <7939e90b-4fe0-d07a-82b4-73cb0f4407fc@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Thu, 09 Mar 2017 13:11:03 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Am 09.03.2017 um 13:33 schrieb Paolo Bonzini: > > > On 09/03/2017 13:16, Wanpeng Li wrote: >> From: Wanpeng Li >> >> Commit b95234c84 (kvm: x86: do not use KVM_REQ_EVENT for APICv interrupt >> injection) disables interrupts before setting vcpu->mode to fix an race: >> >> | The IPI for posted interrupts may be issued between setting vcpu->mode = >> | IN_GUEST_MODE and disabling interrupts. If that happens, >> | kvm_trigger_posted_interrupt returns true, but smp_kvm_posted_intr_ipi >> | doesn't do anything about it. The guest is entered with PIR.ON, but the >> | posted interrupt IPI has not been sent and the interrupt is only delivered >> | to the guest on the next vmentry (if any). >> >> However the race has already been fixed by the side-effects of moving the >> RVI update after IN_GUEST_MODE in the commit: >> >> - The IPI for posted interrupts is issued after setting vcpu->mode = >> IN_GUEST_MODE and disabling interrupts, the posted interrupt IPI is >> delayed until the guest enters non-root mode; it is then trapped by >> the processor causing the interrupt to be injected. >> - The IPI for posted interrupts is issued between setting vcpu->mode = >> IN_GUEST_MODE and disabling interrupts, the movement of the RVI update >> after IN_GUEST_MODE in the commit will catch the new PIR, and set RVI. >> >> This patch tries to reduce the local IRQ disable time by restoring the >> place of disable local IRQ in order to improve host kernel responsibility >> to some degree. > > This should not make any difference; we're talking of a few dozen clock > cycles, and it's quite possible that the extra chance of getting a PI > interrupt in the host negates it. > > Paolo > Also, I think after all the discussion regarding handling vcpu->mode + preemption when it comes to a vcpu_kick(), keeping it that way is way cleaner. -- Thanks, David