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From: Dan Hecht <dhecht@vmware.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Andi Kleen <ak@muc.de>, Chris Wright <chrisw@sous-sol.org>,
	virtualization@lists.osdl.org, xen-devel@lists.xensource.com,
	Ian Pratt <ian.pratt@xensource.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [patch 14/21] Xen-paravirt: Add XEN config options and disable unsupported config options.
Date: Thu, 15 Feb 2007 22:14:45 -0800	[thread overview]
Message-ID: <45D54BD5.8010707@vmware.com> (raw)
In-Reply-To: <20070216022531.570154735@goop.org>

On 02/15/2007 06:25 PM, Jeremy Fitzhardinge wrote:
> The XEN config option enables the Xen paravirt_ops interface, which is
> installed when the kernel finds itself running under Xen. (By some
> as-yet fully defined mechanism, implemented in a future patch.)
> 
> Xen is no longer a sub-architecture, so the X86_XEN subarch config
> option has gone.
> 
> The disabled config options are:
> - PREEMPT: Xen doesn't support it
> - HZ: set to 100Hz for now, to cut down on VCPU context switch rate.
>   This will be adapted to use tickless later.
> - kexec: not yet supported
> 

>  
>  config KEXEC
>  	bool "kexec system call"
> +	depends on !XEN
>  	help
>  	  kexec is a system call that implements the ability to shutdown your
>  	  current kernel, and to start another kernel.  It is like a reboot

>  config DOUBLEFAULT
>  	default y
>  	bool "Enable doublefault exception handler" if EMBEDDED
> +	depends on !XEN
>  	help
>            This option allows trapping of rare doublefault exceptions that
>            would otherwise cause a system to silently reboot. Disabling this


> +config XEN
> +	bool "Enable support for Xen hypervisor"
> +	depends PARAVIRT
> +	default y
> +	help
> +	  This is the Linux Xen port.

>  choice
> -	prompt "Timer frequency"
> +	prompt "Timer frequency" if !XEN
>  	default HZ_250
>  	help
>  	 Allows the configuration of the timer frequency. It is customary
> @@ -49,7 +49,7 @@ endchoice
>  
>  config HZ
>  	int
> -	default 100 if HZ_100
> +	default 100 if HZ_100 || XEN
>  	default 250 if HZ_250
>  	default 300 if HZ_300
>  	default 1000 if HZ_1000


>  config PREEMPT
>  	bool "Preemptible Kernel (Low-Latency Desktop)"
> +	depends on !XEN
>  	help
>  	  This option reduces the latency of the kernel by making
>  	  all kernel code (that is not executing in a critical section)
> 

I hate to sound like a broken record, but this really isn't the right 
way to do this.  If you are going to inhibit config settings when Xen 
support is compiled, then it should be:

config XEN
     depends on !KEXEC && !DOUBLEFAULT && HZ_100 && !PREEMPT

I'm really not trying to make things more difficult for you or for Xen 
users.

One of the primary goals of paravirt-ops is to allow the same kernel 
binary to bind to multiple hypervisor interfaces (and native), at 
runtime.  So, we should assume that kernels which are built with 
CONFIG_PARAVIRT will also enable all the paravirt-ops backends; and this 
is a good thing.

But, the problem is, then, when someone enables paravirt (and all the 
backends, including Xen), you are silently forcing all these options 
off.  Compiling in paravirt support should not change force you into 
using some set of config option; when a paravirt-ops kernel is run on 
native, it should be as fully featured as a non-paravirtops kernel.

So, this should really be solved (in order of preference) by:

1) Complete the xen paravirt-ops backend so it can handle these 
"incompatible" options.  I realize this is just a matter of time (at 
least for most of them, what is your plan for PREEMPT?).

2) Disable the option at runtime only if the kernel is booted on Xen. 
When the kernel is booted on native, lhype, paravirt kvm, vmware, etc it 
should be not be inhibited.  This may not be feasible for all of these 
options, but as Zach pointed out, is easy enough for DOUBLEFAULT.  Maybe 
it can be done for KEXEC?  And HZ is easy to allow too, even though Xen 
will still give interrupts at 100hz -- we do this when you boot on VMI. 
  PREEMPT is probably the only real compile time incompatible options 
with Xen.  You just simply have to change the loop decrementors in your 
timer interrupt.

You basically chose #2 for SMP: while your backend doesn't support it 
yet, it's not harmful to have the config option enabled; you just don't 
allow a second vcpu to startup when running on Xen.

3) Use the config XEN line I suggest above.  The net effect is the same 
as your proposed change (it prevents users from compiling the 
incompatible options together), but at least the user understands what 
is going on, instead of options just silently changing on them.

Dan

  reply	other threads:[~2007-02-16  6:15 UTC|newest]

Thread overview: 100+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-16  2:24 [patch 00/21] Xen-paravirt: Xen guest implementation for paravirt_ops interface Jeremy Fitzhardinge
2007-02-16  2:24 ` [patch 01/21] Xen-paravirt: Fix typo in sync_constant_test_bit()s name Jeremy Fitzhardinge
2007-02-16  2:24 ` [patch 02/21] Xen-paravirt: ignore vgacon if hardware not present Jeremy Fitzhardinge
2007-02-16  2:24 ` [patch 03/21] Xen-paravirt: Add pagetable accessors to pack and unpack pagetable entries Jeremy Fitzhardinge
2007-02-16  2:24 ` [patch 04/21] Xen-paravirt: =================================================================== Jeremy Fitzhardinge
2007-02-16  9:12   ` Andi Kleen
2007-02-16 17:32     ` Jeremy Fitzhardinge
2007-02-16  2:24 ` [patch 05/21] Xen-paravirt: paravirt_ops: hooks to set up initial pagetable Jeremy Fitzhardinge
2007-02-16  2:24 ` [patch 06/21] Xen-paravirt: paravirt_ops: allocate a fixmap slot Jeremy Fitzhardinge
2007-02-16  2:24 ` [patch 07/21] Xen-paravirt: remove ctor for pgd cache Jeremy Fitzhardinge
2007-02-16  6:04   ` Zachary Amsden
2007-02-16  8:39   ` Pekka Enberg
2007-02-16  9:19     ` Nick Piggin
2007-02-16 17:19       ` Jeremy Fitzhardinge
2007-02-16 20:33   ` Christoph Lameter
2007-02-16 23:33     ` Jeremy Fitzhardinge
2007-02-16  2:24 ` [patch 08/21] Xen-paravirt: Allow paravirt backend to choose kernel PMD sharing Jeremy Fitzhardinge
2007-02-16  2:24 ` [patch 09/21] Xen-paravirt: Allow paravirt backend to select PGD allocation alignment Jeremy Fitzhardinge
2007-02-16  2:24 ` [patch 10/21] Xen-paravirt: add hooks to intercept mm creation and destruction Jeremy Fitzhardinge
2007-02-16  6:34   ` Andrew Morton
2007-02-16  6:55     ` Jeremy Fitzhardinge
2007-02-22 20:16   ` Pavel Machek
2007-02-22 20:21     ` Jeremy Fitzhardinge
2007-02-22 20:23     ` Jeremy Fitzhardinge
2007-02-16  2:25 ` [patch 11/21] Xen-paravirt: Add apply_to_page_range() which applies a function to a pte range Jeremy Fitzhardinge
2007-02-16  6:37   ` Andrew Morton
2007-02-16  7:06     ` Jeremy Fitzhardinge
2007-02-16  7:19       ` Andrew Morton
2007-02-16  7:23         ` Jeremy Fitzhardinge
2007-02-16  7:31     ` Nick Piggin
2007-02-16  2:25 ` [patch 12/21] Xen-paravirt: Allocate and free vmalloc areas Jeremy Fitzhardinge
2007-02-16  6:43   ` Andrew Morton
2007-02-16  7:08     ` Jeremy Fitzhardinge
2007-02-16  7:24       ` Andrew Morton
2007-02-16  7:30         ` Jeremy Fitzhardinge
2007-02-16  7:49           ` Andrew Morton
2007-02-16  9:18           ` Andi Kleen
2007-02-16 11:10             ` [Xen-devel] " Keir Fraser
2007-02-16 11:34               ` Andi Kleen
2007-02-16 16:46             ` Jeremy Fitzhardinge
2007-02-16 17:10               ` [Xen-devel] " Keir Fraser
2007-02-16 17:12                 ` Keir Fraser
2007-02-16 17:27                   ` Jeremy Fitzhardinge
2007-02-16 19:06                     ` Keir Fraser
2007-02-16 19:19                       ` Keir Fraser
2007-02-16 19:26                         ` Jeremy Fitzhardinge
2007-02-16 23:29                           ` Keir Fraser
2007-02-16 23:41                             ` Jeremy Fitzhardinge
2007-02-16 17:26                 ` Hollis Blanchard
2007-02-16  7:48         ` Nick Piggin
2007-02-16  2:25 ` [patch 13/21] Xen-paravirt: Add nosegneg capability to the vsyscall page notes Jeremy Fitzhardinge
2007-02-16  6:06   ` Zachary Amsden
2007-02-16  2:25 ` [patch 14/21] Xen-paravirt: Add XEN config options and disable unsupported config options Jeremy Fitzhardinge
2007-02-16  6:14   ` Dan Hecht [this message]
2007-02-16  7:04     ` Jeremy Fitzhardinge
2007-02-16  7:52       ` Dan Hecht
2007-02-16  8:05         ` Jeremy Fitzhardinge
2007-02-16  8:37           ` Dan Hecht
2007-02-16 10:49             ` Keir Fraser
2007-02-16  7:06     ` Andrew Morton
2007-02-16  7:25       ` Jeremy Fitzhardinge
2007-02-16 10:09         ` Keir Fraser
2007-02-16 10:19           ` [Xen-devel] " Zachary Amsden
2007-02-16 10:44             ` Keir Fraser
2007-02-16 10:55               ` Zachary Amsden
2007-02-17  4:04             ` Rusty Russell
2007-02-16  6:15   ` Zachary Amsden
2007-02-16  7:33     ` Eric W. Biederman
2007-02-16  8:14       ` Ian Campbell
2007-02-16  8:24         ` Eric W. Biederman
2007-02-16  8:31           ` Zachary Amsden
2007-02-16 13:53       ` [Xen-devel] " Gerd Hoffmann
2007-02-18 11:32     ` Avi Kivity
2007-02-16  2:25 ` [patch 16/21] Xen-paravirt: Core Xen implementation Jeremy Fitzhardinge
2007-02-16  2:25 ` [patch 17/21] Xen-paravirt: Add the Xen virtual console driver Jeremy Fitzhardinge
2007-02-16  6:48   ` Andrew Morton
2007-02-16  2:25 ` [patch 18/21] Xen-paravirt: Add Xen grant table support Jeremy Fitzhardinge
2007-02-16  6:52   ` Andrew Morton
2007-02-16  6:58     ` Jeremy Fitzhardinge
2007-02-16  2:25 ` [patch 19/21] Xen-paravirt: Add the Xenbus sysfs and virtual device hotplug driver Jeremy Fitzhardinge
2007-02-16  2:25 ` [patch 20/21] Xen-paravirt: Add Xen virtual block device driver Jeremy Fitzhardinge
2007-02-16  2:25 ` [patch 21/21] Xen-paravirt: Add the Xen virtual network " Jeremy Fitzhardinge
2007-02-16  6:59 ` [patch 00/21] Xen-paravirt: Xen guest implementation for paravirt_ops interface Andrew Morton
2007-02-16  7:20   ` Jeremy Fitzhardinge
2007-02-16 20:49 ` Christoph Lameter
2007-02-16 21:04   ` Zachary Amsden
2007-02-16 21:13     ` Christoph Lameter
2007-02-16 21:48       ` Zachary Amsden
2007-02-16 21:59         ` Christoph Lameter
2007-02-16 22:10           ` Zachary Amsden
2007-02-17 13:51           ` Andi Kleen
2007-02-21 18:37             ` Christoph Lameter
2007-02-21 18:55               ` Zachary Amsden
2007-02-21 20:02               ` Jeremy Fitzhardinge
2007-02-17  5:05         ` Rusty Russell
2007-02-16 23:49   ` Jeremy Fitzhardinge
2007-02-17  4:58   ` Rusty Russell
     [not found] <20070213221729.772002682@goop.org>
     [not found] ` <20070213221830.542511707@goop.org>
2007-02-13 22:53   ` [patch 14/21] Xen-paravirt: Add XEN config options and disable unsupported config options Dan Hecht
2007-02-13 23:29     ` Jeremy Fitzhardinge
2007-02-13 23:58       ` Dan Hecht

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