From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934187AbeBLLbd (ORCPT ); Mon, 12 Feb 2018 06:31:33 -0500 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:50048 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S934034AbeBLLba (ORCPT ); Mon, 12 Feb 2018 06:31:30 -0500 Subject: Re: [PATCH] KVM: lapic: stop advertising DIRECTED_EOI when in-kernel IOAPIC is in use To: Vitaly Kuznetsov , Peter Xu Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= References: <20180209130133.28387-1-vkuznets@redhat.com> <20180211052539.GA31261@xz-mi> <87r2pqxx6r.fsf@vitty.brq.redhat.com> From: Paolo Bonzini Message-ID: <4d3e535c-3f26-0a7e-efd7-fd55fd4928e8@redhat.com> Date: Mon, 12 Feb 2018 12:31:27 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 MIME-Version: 1.0 In-Reply-To: <87r2pqxx6r.fsf@vitty.brq.redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/02/2018 12:30, Vitaly Kuznetsov wrote: >> - Radim's suggestion was to disable DIRECTED_EOI unconditionally but I'm not >> that radical :-) In theory, we may have multiple IOAPICs in userspace in >> future and DIRECTED_EOI can be leveraged. > > I sort of agree on this, especially considering that we already have > IOAPIC version 0x20 support in QEMU already. Yeah, and directed EOI is faster AFAIU. > My thoughts were: with in-kernel IOAPIC nobody does EOI broadcast > disabling today or he's already broken -- there's no way to EOI > level-triggered interrupts. And the patch changes nothing for QEMU > ioapic implementation. > > What I'm not sure about is if we allow migration between in-kernel and > QEMU ioapic implementations. I don't think so but in case we do it is > again already broken because going down from 0x20 to 0x11 will leave you > without EOI register. Migrating from 0x11 to 0x20 should be fine, the > guest will continue without knowing there's a new feature available. No, we don't allow that. Paolo