From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH V3 2/3] kvm: don't register wildcard MMIO EVENTFD on two buses Date: Tue, 01 Sep 2015 12:47:36 +0800 Message-ID: <55E52DE8.4000308@redhat.com> References: <1440493548-2971-1-git-send-email-jasowang@redhat.com> <1440493548-2971-2-git-send-email-jasowang@redhat.com> <20150825144925-mutt-send-email-mst@redhat.com> <55DD4A5A.3080006@redhat.com> <55E3C607.1060005@redhat.com> <20150831102838-mutt-send-email-mst@redhat.com> <55E40A6F.5010003@redhat.com> <20150831143047-mutt-send-email-mst@redhat.com> <55E51C97.3040409@redhat.com> <20150901072708-mutt-send-email-mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: gleb@kernel.org, pbonzini@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, cornelia.huck@de.ibm.com To: "Michael S. Tsirkin" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:50301 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752790AbbIAEro (ORCPT ); Tue, 1 Sep 2015 00:47:44 -0400 In-Reply-To: <20150901072708-mutt-send-email-mst@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 09/01/2015 12:31 PM, Michael S. Tsirkin wrote: > On Tue, Sep 01, 2015 at 11:33:43AM +0800, Jason Wang wrote: >> >> On 08/31/2015 07:33 PM, Michael S. Tsirkin wrote: >>> On Mon, Aug 31, 2015 at 04:03:59PM +0800, Jason Wang wrote: >>>>> >>>>> On 08/31/2015 03:29 PM, Michael S. Tsirkin wrote: >>>>>>>>>>>>>>> Thinking more about this, invoking the 0-length write after >>>>>>>>>>>>>>>>>>>>>>> the != 0 length one would be better: it would mean we only >>>>>>>>>>>>>>>>>>>>>>> handle the userspace MMIO like this. >>>>>>>>>>>>>>> Right. >>>>>>>>>>>>>>> >>>>>>>>>>> Using current unittest. This patch is about 2.9% slower than before, and >>>>>>>>>>> invoking 0-length write after is still 1.1% slower (mmio-datamatch-eventfd). >>>>>>>>>>> >>>>>>>>>>> /patch/result/-+%/ >>>>>>>>>>> /base/2957/0/ >>>>>>>>>>> /V3/3043/+2.9%/ >>>>>>>>>>> /V3+invoking != 0 length first/2990/+1.1%/ >>>>>>>>>>> >>>>>>>>>>> So looks like the best method is not searching KVM_FAST_MMIO_BUS during >>>>>>>>>>> KVM_MMIO_BUS. Instead, let userspace to register both datamatch and >>>>>>>>>>> wildcard in this case. Does this sound good to you? >>>>>>> No - we can't change userspace. >>>>> Actually, the change was as simple as following. So I don't get the >>>>> reason why. >>> Because it's too late - we committed to a specific userspace ABI >>> when this was merged in kernel, we must maintain it. >> Ok ( Though I don't think it has real users for this now because it was >> actually broken). > It actually worked most of the time - you only trigger a use after free > on deregister. > It doesn't work for amd and intel machine without ept. >>> Even if I thought yours is a good API (and I don't BTW - it's exposing >>> internal implementation details) it's too late to change it. >> I believe we should document the special treatment in kernel of zero >> length mmio eventfd in api.txt? If yes, is this an exposing? If not, how >> can userspace know the advantages of this and use it? For better API, >> probably we need another new flag just for fast mmio and obsolete >> current one by failing the assigning for zero length mmio eventfd. > I sent a patch to update api.txt already as part of > kvm: add KVM_CAP_IOEVENTFD_PF capability. > I should probably split it out. > > Sorry, I don't think the api change you propose makes sense - just fix the > crash in the existing one. > Ok, so I believe the fix should go: - having two ioeventfds when we want to assign zero length mmio eventfd - change the kvm_io_bus_sort_cmp() and can handle zero length correctly What's your thought?