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From: Don Dutile <ddutile@redhat.com>
To: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>,
	"jeremy@goop.org" <jeremy@goop.org>,
	"xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
	"stefano@stabellini.net" <stefano@stabellini.net>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"sheng@linux.intel.com" <sheng@linux.intel.com>
Subject: Re: [Xen-devel] Re: [PATCH 12/13] Unplug emulated disks and nics
Date: Thu, 08 Jul 2010 17:59:27 -0400	[thread overview]
Message-ID: <4C364A3F.8020900@redhat.com> (raw)
In-Reply-To: <1278624583.12109.15.camel@localhost.localdomain>

Ian Campbell wrote:
> On Thu, 2010-07-08 at 20:57 +0100, Don Dutile wrote:
>> I guess what I'm wondering is why not set xen_emul_unplug to ignore by
>> default (static int xen_emul_unplug=XEN_UNPLUG_IGNORE), which handles
>> the case I mentioned (just take existing guest config file as is, no edits,
>> pre-pv-hvm added to guest kernel),  and if person edits config file to 
>> change boot device to xvda, they would then edit the config to add
>> -x xen_emul_unplug=[all|ide-disks|...]  as well.
> 
> Can you guarantee that nobody is running an HVM guest today with a
> configuration file that specifies xvda (I believe it would work)? In
> other words can you be sure that defaulting to XEN_UNPLUG_IGNORE is
> _always_ going to be safe? Not just on RHEL hosts and with
> configurations generated by the RH tools or according to the RH docs but
> on any host with any (possibly hand-crafted) configuration?
> 
No, you have a valid point.  We have pv-on-hvm support for rhel3->rhel5
HVM guests, and they support xvda on boot devices (once initrd is rebuilt),
so it's possible to have that config (spec'd) as well, and someone
to copy & edit it for use on a more current disk image w/relatively current
kernel.
But I'm considering 2.6.32+ HVM guests that didn't have xvda spec'd in
the boot path ever, and are upgraded to a post-2.6.32 kernel that has
pv-on-hvm added to it.

> Any guest which uses xvda in its configuration file today will be using
> emulated devices but I think that with Stefano's patch and your proposed
> change in default on a Xen system without support for unplug will start
> using PV devices without unplugging the emulated ones first.
> 
Well, Stefano requires the admin to add unplug switch to kernel cmd line,
so I don't see the harm in defaulting to unplug... 

> I don't think there is any way for a guest running on a platform which
> does not support the unplug protocol to know automatically if it is safe
> to use the PV devices or not, therefore we have to err on the side of
> caution and ask users with such systems who know that their
> configuration is safe to explicitly request PV devices by using the
> command line option. Doing anything else is taking risks with people's
> data.
> 
> Ian.
> 
> 
Either way, the user/admin has to add cmdline to unplug to be safe.
I don't see how defaulting to UNPLUG_IGNORE changes that requirement.
... or am I missing a case? -- ah, if IGNORE isn't spec'd, pvhvm just won't
be configured in, and blkfront wont run, and cant have blkfront & ide
accessing the same device.... is that the case I'm missing ?

- Don



  reply	other threads:[~2010-07-08 22:00 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-06-21 16:12 [PATCH 0/12] PV on HVM Xen Stefano Stabellini
2010-06-21 16:12 ` Stefano Stabellini
2010-06-21 16:13 ` [PATCH 01/13] Add support for hvm_op stefano
2010-06-21 16:13 ` [PATCH 02/13] early PV on HVM stefano
2010-06-21 16:13 ` [PATCH 03/13] evtchn delivery " stefano
2010-06-21 16:13 ` [PATCH 04/13] Xen PCI platform device driver stefano
2010-06-30 17:55   ` Konrad Rzeszutek Wilk
2010-06-30 17:55     ` Konrad Rzeszutek Wilk
2010-07-01 11:38     ` Stefano Stabellini
2010-06-21 16:13 ` [PATCH 05/13] Add suspend\resume support for PV on HVM guests stefano
2010-06-21 16:14 ` [PATCH 06/13] Allow xen platform pci device to be compiled as a module stefano
2010-06-21 16:14 ` [PATCH 0/12] PV on HVM Xen Stefano Stabellini
2010-06-21 16:14   ` Stefano Stabellini
2010-06-21 16:14 ` [PATCH 07/13] Fix find_unbound_irq in presence of ioapic irqs stefano
2010-06-21 16:14 ` [PATCH 08/13] Fix possible NULL pointer dereference in print_IO_APIC stefano
2010-06-21 16:14 ` [PATCH 09/13] __setup_vector_irq: handle NULL chip_data stefano
2010-06-21 16:14 ` [PATCH 10/13] Do not try to disable hpet if it hasn't been initialized before stefano
2010-06-30 17:53   ` Konrad Rzeszutek Wilk
2010-06-30 17:53     ` Konrad Rzeszutek Wilk
2010-07-09  1:05     ` john stultz
2010-06-30 21:24   ` Venkatesh Pallipadi
2010-06-30 21:24     ` Venkatesh Pallipadi
2010-07-02 10:44   ` Paolo Bonzini
2010-07-02 10:44     ` Paolo Bonzini
2010-06-21 16:14 ` [PATCH 11/13] Use xen_vcpuop_clockevent, xen_clocksource and xen wallclock stefano
2010-07-01 19:41   ` Don Dutile
2010-07-02 17:17     ` Stefano Stabellini
2010-07-02 17:17       ` Stefano Stabellini
2010-06-21 16:14 ` [PATCH 12/13] Unplug emulated disks and nics stefano
2010-07-01 19:41   ` Don Dutile
2010-07-01 19:41     ` Don Dutile
2010-07-05 11:58     ` Stefano Stabellini
2010-07-07 20:01       ` Don Dutile
2010-07-08 13:13         ` Stefano Stabellini
2010-07-08 13:13           ` Stefano Stabellini
2010-07-08 19:57           ` [Xen-devel] " Don Dutile
2010-07-08 21:29             ` Ian Campbell
2010-07-08 21:29               ` Ian Campbell
2010-07-08 21:59               ` Don Dutile [this message]
2010-07-09  8:02                 ` [Xen-devel] " Ian Campbell
2010-07-09  8:02                   ` Ian Campbell
2010-07-09 10:54                   ` Stefano Stabellini
2010-07-09 10:54                     ` Stefano Stabellini
2010-07-09 13:42                     ` [Xen-devel] " Don Dutile
2010-06-21 16:14 ` [PATCH 13/13] Call HVMOP_pagetable_dying on exit_mmap stefano
2010-06-30 17:56 ` [PATCH 0/12] PV on HVM Xen Konrad Rzeszutek Wilk
2010-07-01 11:38   ` Stefano Stabellini
2010-07-01 19:43     ` [Xen-devel] " Dan Magenheimer
2010-07-02 10:43       ` Stefano Stabellini
2010-07-02 16:08         ` Dan Magenheimer
2010-07-02 17:14           ` Stefano Stabellini
2010-07-02 19:49             ` Dan Magenheimer
2010-07-02 19:49               ` Dan Magenheimer

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