Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753990Ab2JVMz6 (ORCPT ); Mon, 22 Oct 2012 08:55:58 -0400 Received: from mx1.redhat.com ([209.132.183.28]:63073 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753849Ab2JVMz4 (ORCPT ); Mon, 22 Oct 2012 08:55:56 -0400 Message-ID: <5085423C.2020908@redhat.com> Date: Mon, 22 Oct 2012 14:55:24 +0200 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120911 Thunderbird/15.0.1 MIME-Version: 1.0 To: Gleb Natapov CC: Jan Kiszka , Xiao Guangrong , Marcelo Tosatti , LKML , KVM Subject: Re: [PATCH] KVM: x86: fix vcpu->mmio_fragments overflow 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> <20121022125301.GS29310@redhat.com> In-Reply-To: <20121022125301.GS29310@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1947 Lines: 41 On 10/22/2012 02:53 PM, Gleb Natapov wrote: > 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. It's time where the guest cannot take interrupts, and time in a high priority guest thread that is spent processing low guest priority requests. -- error compiling committee.c: too many arguments to function -- 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/