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Re: [Xen-devel] [PATCH 1/2] xl: neuter vcpu-set --ignore-host.

On 26/09/13 13:48, Konrad Rzeszutek Wilk wrote:
On Thu, Sep 26, 2013 at 10:06:31AM +0100, Ian Campbell wrote:
On Wed, 2013-09-25 at 16:40 -0400, Konrad Rzeszutek Wilk wrote:
When Xen 4.3 was released we had a discussion whether we should
allow the vcpu-set command to allow the user to set more than
physical CPUs for a guest (it didn't). The author brought up:
  - Xend used to do it,
IMHO xend is buggy here. If it were being maintained I encourage a patch
to file this particular sharp edge off.

  - If a user wants to do it, let them do it,
We do, we have an option for those who know what they are doing to use
in the tiny minority of cases where they need to do this.

  - The original author of the change did not realize the
    side-effect his patch caused this and had no intention of changing it.
a happy accident then.

  - The user can already boot a massively overcommitted guest by
    having a large 'vcpus=' value in the guest config and we allow
IMHO this is an xl bug, I'd be happy to see a patch to fix this and
require and override here too.
I think I posted one some time ago, but I don't recall anybody
commenting on it. Will repost it.
Since we were close to the release we added --ignore-host parameter
as a mechanism for a user to still set more vCPUs that the physical
machine as a stop-gate.

This patch keeps said option but neuters the check so that we
can overcommit. In other words - by default the user is
allowed to set as many vCPUs as they would like.
and why would a naive user want to do this? non-naive users can use the
option if this is what they really want, and are probably grateful for
the catch if they didn't intend to overcommit, which is almost always
even for expert users.

This change need far better rationalisation than "because xend did it"
and "because we can". IMHO.
I am going to defer to George here. His viewpoint (I am going to
probably mangle it up) was that - if the user wants to do, let him/her
do it without us putting obstacles.

And I think Ian Jackson was ambivalent here and was deferring to George.

So I've gone back and read the original thread, and what I actually said was:

"So I think the right thing to do long-term is to make it possible to do in xl. Having a "seatbelt" restriction by default that can be overridden would be OK with me, but I think a warning message when vcpus > pcpus would suffice."

And my summary of mine and IanC's positions at the time (which IanC did not dispute) was:

"We both agree that "vcpus > pcpus" is a bad configuration. I think ideally we should support it (because administrators should be allowed to shoot themselves in the foot) and Ian[C] seems to be making the case that we shouldn't support it."

IanJ, as I understood him, agreed with me that it should be *possible*.

As IanC points out, it is possible -- you just have to add "--ignore-host".

So given what all of us think, keeping the "seatbelt" is probably the best compromise. IanC is happy that a hapless user will not accidentally shoot his own foot, and IanJ and I are happy that a skilled user can shoot her own foot if she really wants to.


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