From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [RFC PATCH 0/3] generic hypercall support Date: Mon, 11 May 2009 12:51:12 -0500 Message-ID: <4A086590.4040602@codemonkey.ws> References: <20090505132005.19891.78436.stgit@dev.haskins.net> <4A0040C0.1080102@redhat.com> <4A0041BA.6060106@novell.com> <4A004676.4050604@redhat.com> <4A0049CD.3080003@gmail.com> <20090505231718.GT3036@sequoia.sous-sol.org> <4A010927.6020207@novell.com> <20090506072212.GV3036@sequoia.sous-sol.org> <4A018DF2.6010301@novell.com> <4A02D40D.7060307@redhat.com> <4A0448DF.90705@codemonkey.ws> <4A0570B1.30401@novell.com> <4A071F1A.1090702@codemonkey.ws> <4A0824C2.4000109@gmail.com> <1242059712.29194.12.camel@slate.austin.ibm.com> <4A085B06.4080803@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Hollis Blanchard , Gregory Haskins , Gregory Haskins , Chris Wright , linux-kernel@vger.kernel.org, kvm@vger.kernel.org To: Avi Kivity Return-path: Received: from an-out-0708.google.com ([209.85.132.244]:49021 "EHLO an-out-0708.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752392AbZEKRvO (ORCPT ); Mon, 11 May 2009 13:51:14 -0400 In-Reply-To: <4A085B06.4080803@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: Avi Kivity wrote: > Hollis Blanchard wrote: >> I haven't been following this conversation at all. With that in mind... >> >> AFAICS, a hypercall is clearly the higher-performing option, since you >> don't need the additional memory load (which could even cause a page >> fault in some circumstances) and instruction decode. That said, I'm >> willing to agree that this overhead is probably negligible compared to >> the IOp itself... Ahmdal's Law again. >> > > It's a question of cost vs. benefit. It's clear the benefit is low > (but that doesn't mean it's not worth having). The cost initially > appeared to be very low, until the nested virtualization wrench was > thrown into the works. Not that nested virtualization is a reality -- > even on svm where it is implemented it is not yet production quality > and is disabled by default. > > Now nested virtualization is beginning to look interesting, with > Windows 7's XP mode requiring virtualization extensions. Desktop > virtualization is also something likely to use device assignment > (though you probably won't assign a virtio device to the XP instance > inside Windows 7). > > Maybe we should revisit the mmio hypercall idea again, it might be > workable if we find a way to let the guest know if it should use the > hypercall or not for a given memory range. > > mmio hypercall is nice because > - it falls back nicely to pure mmio > - it optimizes an existing slow path, not just new device models > - it has preexisting semantics, so we have less ABI to screw up > - for nested virtualization + device assignment, we can drop it and > get a nice speed win (or rather, less speed loss) If it's a PCI device, then we can also have an interrupt which we currently lack with vmcall-based hypercalls. This would give us guestcalls, upcalls, or whatever we've previously decided to call them. Regards, Anthony Liguori