Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752784AbcD2HlU (ORCPT ); Fri, 29 Apr 2016 03:41:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46310 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752022AbcD2HlR (ORCPT ); Fri, 29 Apr 2016 03:41:17 -0400 Date: Fri, 29 Apr 2016 08:41:14 +0100 From: "Richard W.M. Jones" To: "Matwey V. Kornilov" Cc: Greg KH , Jiri Slaby , Peter Hurley , andriy.shevchenko@linux.intel.com, phillip.raffeck@fau.de, anton.wuerfel@fau.de, yamada.masahiro@socionext.com, valentinrothberg@gmail.com, linux-serial@vger.kernel.org, linux-kernel , ddutile@redhat.com Subject: Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs. Message-ID: <20160429074114.GC3826@redhat.com> References: <1461881913-23967-1-git-send-email-rjones@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-12-10) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1664 Lines: 35 On Fri, Apr 29, 2016 at 10:01:08AM +0300, Matwey V. Kornilov wrote: > 2016-04-29 1:18 GMT+03:00 Richard W.M. Jones : > > [This is an opinionated patch, mainly for discussion.] > > > > I'm trying to reduce the time taken in the kernel in initcalls, with > > my aim being to reduce the current ~700ms spent in initcalls before > > userspace, down to something like 100ms. All times on my Broadwell-U > > laptop, under virtualization. The purpose of this is to be able to > > launch VMs around containers with minimal overhead, like Intel Clear > > Containers, but using standard distro kernels and qemu. > > > > Currently the kernel spends 25ms inspecting the UART that we passed to > > it from qemu to find out whether it's an 8250/16550/16550A perhaps > > with a non-working FIFO or other quirks. Well, it isn't -- it's a > > working emulated 16550A, with a FIFO and no quirks, and if it isn't, > > we should fix qemu. > > > > So the patch detects if we're running virtualized (perhaps it should > > only check for qemu/KVM?) and if so, shortcuts the tests. > > Does anybody know, whether it is possible to pass through real > hardware serial port to a guest? It seems to be as simple as to pass > through an interrupt and memory IO ports. In theory it seems like something VFIO could do. Passing something as low performance as a serial port through would seem to make little sense though. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html