From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33 Date: Tue, 22 Dec 2009 21:25:20 +0200 Message-ID: <4B311D20.7020400@redhat.com> References: <4B1D4F29.8020309@gmail.com> <20091218215107.GA14946@elte.hu> <4B2F9582.5000002@gmail.com> <20091222075742.GB26467@elte.hu> <4B3103B4.4070708@gmail.com> <4B3115C1.4070303@redhat.com> <4B311ADA.4000700@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Ingo Molnar , kvm@vger.kernel.org, Andrew Morton , torvalds@linux-foundation.org, "linux-kernel@vger.kernel.org" , netdev@vger.kernel.org, "alacrityvm-devel@lists.sourceforge.net" To: Gregory Haskins Return-path: Received: from mx1.redhat.com ([209.132.183.28]:31786 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755201AbZLVT01 (ORCPT ); Tue, 22 Dec 2009 14:26:27 -0500 In-Reply-To: <4B311ADA.4000700@gmail.com> Sender: netdev-owner@vger.kernel.org List-ID: On 12/22/2009 09:15 PM, Gregory Haskins wrote: > On 12/22/09 1:53 PM, Avi Kivity wrote: > >> I asked why the irqfd/ioeventfd mechanisms are insufficient, and you did not reply. >> >> > BTW: the ioeventfd issue just fell through the cracks, so sorry about > that. Note that I have no specific issue with irqfd ever since the > lockless IRQ injection code was added. > > ioeventfd turned out to be suboptimal for me in the fast path for two > reasons: > > 1) the underlying eventfd is called in atomic context. I had posted > patches to Davide to address that limitation, but I believe he rejected > them on the grounds that they are only relevant to KVM. > If you're not doing something pretty minor, you're better of waking up a thread (perhaps _sync if you want to keep on the same cpu). With the new user return notifier thingie, that's pretty cheap. > 2) it cannot retain the data field passed in the PIO. I wanted to have > one vector that could tell me what value was written, and this cannot be > expressed in ioeventfd. > > It would be easier to add data logging support to ioeventfd, if it was needed that badly. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.