From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [PATCH] KVM: emulator: do not needlesly sync registers from emulator ctxt to vcpu Date: Wed, 30 Mar 2011 12:47:27 +0200 Message-ID: <20110330104727.GS7766@redhat.com> References: <20110329120838.GQ7766@redhat.com> <4D93083D.6000806@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: mtosatti@redhat.com, kvm@vger.kernel.org To: Avi Kivity Return-path: Received: from mx1.redhat.com ([209.132.183.28]:26225 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754647Ab1C3Kr3 (ORCPT ); Wed, 30 Mar 2011 06:47:29 -0400 Received: from int-mx12.intmail.prod.int.phx2.redhat.com (int-mx12.intmail.prod.int.phx2.redhat.com [10.5.11.25]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id p2UAlSFK031225 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Wed, 30 Mar 2011 06:47:28 -0400 Content-Disposition: inline In-Reply-To: <4D93083D.6000806@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Wed, Mar 30, 2011 at 12:38:53PM +0200, Avi Kivity wrote: > On 03/29/2011 02:08 PM, Gleb Natapov wrote: > >Currently we sync registers back and forth before/after exiting > >to userspace for IO, but during IO device model shouldn't need to > >read/write the registers, so we can as well skip those sync points. The > >only exaception is broken vmware backdor interface. The new code sync > >registers content during IO only if registers are read from/written to > >by userspace in the middle of the IO operation and this almost never > >happens in practise. > > While this is a nice idea, how much does it save in practice? It > does introduce more complexity. > I haven't measured, but can try to do so. It eliminates two copies of all registers on each MMIO/PIO read. I expect this to be measurable in workloads that do many such reads. -- Gleb.