From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 2/3] KVM: Emulate MSI-X table in kernel Date: Mon, 17 Jan 2011 14:51:37 +0200 Message-ID: <4D343B59.4070507@redhat.com> References: <1294309185-21417-1-git-send-email-sheng@linux.intel.com> <1294309185-21417-3-git-send-email-sheng@linux.intel.com> <20110117115447.GA8145@amt.cnet> <201101172018.22478.sheng@linux.intel.com> <4D3433A3.2010305@redhat.com> <20110117124816.GB8985@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Sheng Yang , "Michael S. Tsirkin" , kvm@vger.kernel.org To: Marcelo Tosatti Return-path: Received: from mx1.redhat.com ([209.132.183.28]:39512 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751798Ab1AQMvm (ORCPT ); Mon, 17 Jan 2011 07:51:42 -0500 In-Reply-To: <20110117124816.GB8985@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: On 01/17/2011 02:48 PM, Marcelo Tosatti wrote: > On Mon, Jan 17, 2011 at 02:18:43PM +0200, Avi Kivity wrote: > > On 01/17/2011 02:18 PM, Sheng Yang wrote: > > >> > + > > >> > + if (copy_to_user((void __user *)(entry_base + offset), val, len)) > > >> > + goto out; > > >> > > >> Instead of copying to/from userspace (which is subject to swapin, > > >> unexpected values), you could include the guest written value in a > > >> kvm_run structure, along with address. Qemu-kvm would use that to > > >> synchronize its copy of the table, on KVM_EXIT_MSIX_ROUTING_UPDATE exit. > > > > > >We want to acelerate MSI-X mask bit accessing, which won't exit to userspace in > > >the most condition. That's the cost we want to optimize. Also it's possible to > > >userspace to read the correct value of MMIO(but mostly userspace can't write to it > > >in order to prevent synchronize issue). > > > > It's also good to have the values in just one place; using userspace > > makes it easy for both the kernel and userspace to see the values > > (and set them after migration, if/when we extend this to virtio). > > Right, thats an advantage, but: > > - How can userspace ever synchronize with updates by the kernel > to the MSI-X entry? What a value is written by the guest, which kvm cannot handle itself (i.e. a change to anything other than the mask bit), we exit with the table and entry ids, so userspace can reread them. > - Reading/writing to the userspace area must be done carefully, > values must be validated before used. True every time... > - Swapping issue (minor?). I don't see the issue... just like any part of qemu that may be swapped out, blocking the vcpu thread. -- error compiling committee.c: too many arguments to function