Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754017AbcD2QDA (ORCPT ); Fri, 29 Apr 2016 12:03:00 -0400 Received: from mx1.redhat.com ([209.132.183.28]:51893 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753725AbcD2QC7 (ORCPT ); Fri, 29 Apr 2016 12:02:59 -0400 Date: Fri, 29 Apr 2016 17:02:57 +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: <20160429160256.GE28599@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> <20160429153757.GE3826@redhat.com> <20160429155413.GA23477@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160429155413.GA23477@kroah.com> User-Agent: Mutt/1.5.20 (2009-12-10) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Fri, 29 Apr 2016 16:02:58 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1335 Lines: 30 On Fri, Apr 29, 2016 at 08:54:13AM -0700, Greg KH wrote: > You are trying to take a generalized kernel and somehow "know" about the > hardware ahead of time it is going to run on. That seems like two > conflicting requirements, don't you agree? We would have the 8250 serial port in any kernel. Even if Fedora kernel maintainers allowed us to have specialized kernels for each purpose, I would use the simple ISA serial port here because it allows us to capture debug messages very early in the boot. Alternatives like virtio-console don't allow that. The kernel does know what hardware it's running on - via the CPUID hypervisor leaf. It's also possible for us to tell the kernel about the hardware using the command line, ACPI[*], DT, etc. I'd really like to tell the kernel this is a 16550A, not broken, you don't need to spend time testing that. There is prior art here: no_timer_check & lpj=.. Rich. [*] Although ACPI is really slow, adding another 190ms, and for this reason I have disabled it for now, but not investigated why it's so slow. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org