Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753809Ab2JVMxI (ORCPT ); Mon, 22 Oct 2012 08:53:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:17632 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753452Ab2JVMxH (ORCPT ); Mon, 22 Oct 2012 08:53:07 -0400 Date: Mon, 22 Oct 2012 14:53:01 +0200 From: Gleb Natapov To: Jan Kiszka Cc: Avi Kivity , Xiao Guangrong , Marcelo Tosatti , LKML , KVM Subject: Re: [PATCH] KVM: x86: fix vcpu->mmio_fragments overflow Message-ID: <20121022125301.GS29310@redhat.com> References: <5081033C.4060503@linux.vnet.ibm.com> <20121022091615.GG29310@redhat.com> <50852972.305@linux.vnet.ibm.com> <20121022112314.GO29310@redhat.com> <50852F9C.9020808@siemens.com> <20121022114311.GQ29310@redhat.com> <508531E1.2030307@siemens.com> <508539A8.40404@redhat.com> <50853FF1.8010809@siemens.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50853FF1.8010809@siemens.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1669 Lines: 36 On Mon, Oct 22, 2012 at 02:45:37PM +0200, Jan Kiszka wrote: > On 2012-10-22 14:18, Avi Kivity wrote: > > On 10/22/2012 01:45 PM, Jan Kiszka wrote: > > > >>> Indeed. git pull, recheck and call for kvm_flush_coalesced_mmio_buffer() > >>> is gone. So this will break new userspace, not old. By global you mean > >>> shared between devices (or memory regions)? > >> > >> Yes. We only have a single ring per VM, so we cannot flush multi-second > >> VGA access separately from other devices. In theory solvable by > >> introducing per-region rings that can be driven separately. > > > > But in practice unneeded. Real time VMs can disable coalescing and not > > use planar VGA modes. > > A) At least right now, we do not differentiate between the VGA modes and > if flushing is needed. So that device is generally taboo for RT cores of > the VM. > B) We need to disable coalescing in E1000 as well - if we want to use > that model. > C) Gleb seems to propose using coalescing far beyond those two use cases. > Since the userspace change is needed the idea is dead, but if we could implement it I do not see how it can hurt the latency if it would be the only mechanism to use coalesced mmio buffer. Checking that the ring buffer is empty is cheap and if it is not empty it means that kernel just saved you a lot of 8 bytes exists so even after iterating over all the entries there you still saved a lot of time. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/