Prev: [PATCH] perf wrong branches event on AMD
Next: perf wrong branches event on AMD
From: Ian Campbell on 8 Jul 2010 17:40
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?

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.

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.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Don Dutile on 8 Jul 2010 18:10
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


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Ian Campbell on 9 Jul 2010 04:10
On Thu, 2010-07-08 at 22:59 +0100, Don Dutile wrote:
> 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.

An HVM guest could have any kernel version, even one prior to 2.6.32,
and be updated to a post-2.6.32 with PV-on-HVM, we cannot restrict
ourselves to just 2.6.32+ kernels (not that I think it makes a
difference anyhow).

>
> > 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,

In the case where the host platform does not support the unplug protocol
this is correct and requiring explicit admin action to allow the PV
frontends to activate is the only safe option WRT the users data.

However if the host platform does support the unplug protocol then this
is incorrect. In that case the default (in the absence of the command
line option) is to automatically unplug any device for which a PV driver
is available and so no command line option will be required in the
common case. (see xen_unplug_emulated_devices() under the comment "Set
the default value of xen_emul_unplug depending on...")

> so I don't see the harm in defaulting to unplug...

As I described in my previous mail this is unsafe on host platforms
which do not support unplug. As I describe above it is unnecessary on
host platforms which do support unplug

> Either way, the user/admin has to add cmdline to unplug to be safe.

Not true, on a recent Xen system which supports the unplug protocol then
devices will be unplugged automatically without an additional command
line option.

Ian.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Stefano Stabellini on 9 Jul 2010 07:00
On Fri, 9 Jul 2010, Ian Campbell wrote:
> >
> > > 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,
>
> In the case where the host platform does not support the unplug protocol
> this is correct and requiring explicit admin action to allow the PV
> frontends to activate is the only safe option WRT the users data.
>
> However if the host platform does support the unplug protocol then this
> is incorrect. In that case the default (in the absence of the command
> line option) is to automatically unplug any device for which a PV driver
> is available and so no command line option will be required in the
> common case. (see xen_unplug_emulated_devices() under the comment "Set
> the default value of xen_emul_unplug depending on...")
>

that's right: on host platforms supporting unplug no command line options
are required; on the other hand if the host platform does not support
unplug then xen_emul_unplug=ignore is required to use PV drivers anyway.


> > so I don't see the harm in defaulting to unplug...
>
> As I described in my previous mail this is unsafe on host platforms
> which do not support unplug. As I describe above it is unnecessary on
> host platforms which do support unplug
>

yep, keep in mind the copy and paste example.



If you are happy about the patch, I'll include it in my next version.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Don Dutile on 9 Jul 2010 09:50
Stefano Stabellini wrote:
> On Fri, 9 Jul 2010, Ian Campbell wrote:
>>>> 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,
>> In the case where the host platform does not support the unplug protocol
>> this is correct and requiring explicit admin action to allow the PV
>> frontends to activate is the only safe option WRT the users data.
>>
>> However if the host platform does support the unplug protocol then this
>> is incorrect. In that case the default (in the absence of the command
>> line option) is to automatically unplug any device for which a PV driver
>> is available and so no command line option will be required in the
>> common case. (see xen_unplug_emulated_devices() under the comment "Set
>> the default value of xen_emul_unplug depending on...")
>>
>
> that's right: on host platforms supporting unplug no command line options
> are required; on the other hand if the host platform does not support
> unplug then xen_emul_unplug=ignore is required to use PV drivers anyway.
>
>
I see by looking at xen_unplug_emulated_devices() closer, that the default
unplugs emulated ide & emulated nic:
if (xen_platform_pci_enabled && !xen_emul_unplug)

I had to work around the fact that xen_platform_pci_enabled would
not be set on rhel5-dom0 (no emul-unplug), so I hadn't looked at this default
so closely.


>>> so I don't see the harm in defaulting to unplug...
>> As I described in my previous mail this is unsafe on host platforms
>> which do not support unplug. As I describe above it is unnecessary on
>> host platforms which do support unplug
>>
>
> yep, keep in mind the copy and paste example.
>
>
>
> If you are happy about the patch, I'll include it in my next version.
>

Yes, the patch seems like a reasonable solution for running on older qemu-dm's.

Thanks for the clarification(s).

- Don
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
First  |  Prev  | 
Pages: 1 2
Prev: [PATCH] perf wrong branches event on AMD
Next: perf wrong branches event on AMD