From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [patch 0/3] RFC: provide synchronous registers in kvm_run Date: Thu, 22 Dec 2011 14:54:53 +0200 Message-ID: <4EF3289D.3080903@redhat.com> References: <20111222115646.248800653@de.ibm.com> <4EF32421.8000303@redhat.com> <4EF32747.4050809@de.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Marcelo Tossati , Carsten Otte , Alexander Graf , Jens Freimann , Cornelia Huck , Heiko Carstens , Martin Schwidefsky , KVM To: Christian Borntraeger Return-path: Received: from mx1.redhat.com ([209.132.183.28]:16925 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751608Ab1LVMzE (ORCPT ); Thu, 22 Dec 2011 07:55:04 -0500 In-Reply-To: <4EF32747.4050809@de.ibm.com> Sender: kvm-owner@vger.kernel.org List-ID: On 12/22/2011 02:49 PM, Christian Borntraeger wrote: > > Interesting. Other archs emulate everything to do with registers in the > > kernel, so this is not a fast path. > > > > What workload does this benefit? > > My main concern was the prefix register (this is a per cpu register that > defines the address of two pages that are swapped with the pages at 0 for this cpu). > SMP on s390 is done that way (e.g. interrupt things are stored in page 0 for this cpu) > The storage that qemu sees is storage without prefix. For architecture compliance > we actually must check _every_ memory access if it hits the prefix/swpa area and > the add/subtract the prefix value. Those are only memory accesses coming from the cpu, yes? Why does userspace have to access them at all? I imagine DMA ignores it completely since it doesn't come from the cpu. > I just added the ability to share other registers later after some discussions > with Alexander Graf because it seems to be doable for almost no cost. I doubt other archs will benefit, but it makes sense to reserve the space generically as you did. -- error compiling committee.c: too many arguments to function