Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753855AbcD2PiD (ORCPT ); Fri, 29 Apr 2016 11:38:03 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47699 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752928AbcD2PiA (ORCPT ); Fri, 29 Apr 2016 11:38:00 -0400 Date: Fri, 29 Apr 2016 16:37:57 +0100 From: "Richard W.M. Jones" To: Greg KH Cc: jslaby@suse.com, peter@hurleysoftware.com, andriy.shevchenko@linux.intel.com, phillip.raffeck@fau.de, anton.wuerfel@fau.de, yamada.masahiro@socionext.com, matwey@sai.msu.ru, valentinrothberg@gmail.com, linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org, ddutile@redhat.com Subject: Re: [PATCH] 8250: Hypervisors always export working 16550A UARTs. Message-ID: <20160429153757.GE3826@redhat.com> References: <1461881913-23967-1-git-send-email-rjones@redhat.com> <1461881913-23967-2-git-send-email-rjones@redhat.com> <20160428225633.GA9134@kroah.com> <20160429081006.GD3826@redhat.com> <20160429151635.GB16895@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160429151635.GB16895@kroah.com> 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: 2397 Lines: 54 On Fri, Apr 29, 2016 at 08:16:35AM -0700, Greg KH wrote: > On Fri, Apr 29, 2016 at 09:10:06AM +0100, Richard W.M. Jones wrote: > > On Thu, Apr 28, 2016 at 03:56:33PM -0700, Greg KH wrote: > > > On Thu, Apr 28, 2016 at 11:18:33PM +0100, Richard W.M. Jones wrote: > > > > Currently autoconf spends 25ms (on my laptop) testing if the UART > > > > exported to it by KVM is an 8250 without FIFO and/or with strange > > > > quirks, which it obviously isn't. Assume it is exported to us by a > > > > hypervisor, it's a normal, working 16550A. > > > > > > > > Signed-off-by: Richard W.M. Jones > > > > --- > > > > drivers/tty/serial/8250/8250_port.c | 7 +++++++ > > > > 1 file changed, 7 insertions(+) > > > > > > > > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c > > > > index 00ad2637..de19924 100644 > > > > --- a/drivers/tty/serial/8250/8250_port.c > > > > +++ b/drivers/tty/serial/8250/8250_port.c > > > > @@ -1171,6 +1171,13 @@ static void autoconfig(struct uart_8250_port *up) > > > > if (!port->iobase && !port->mapbase && !port->membase) > > > > return; > > > > > > > > + /* Hypervisors always export working 16550A devices. */ > > > > + if (cpu_has_hypervisor) { > > > > + up->port.type = PORT_16550A; > > > > + up->capabilities |= UART_CAP_FIFO; > > > > + return; > > > > + } > > > > > > Have you audited vmware, virtualbox, and everyone else that provides a > > > virtual uart device that it will work properly here? > > > > > > qemu isn't all the world :) > > > > Attached below is a slightly different approach. If the user passes a > > special flag on the kernel command line then we force 16550A and avoid > > the 25ms delay. Since the user chooses the flag, any concerns about > > the behaviour of the hypervisor or use of VFIO should be moot. > > No, no more module parameters, that's crazy, what happens when you have > 64 serial ports in a system, which one is this option for? In this (very special) case, the domain is running under qemu and I know it only has one serial port that doesn't need probing. What's the right way to avoid this 25ms delay? 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