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[Xen-devel] per-domain configuration and inappropriate use of globals
- To: Xen-devel List <xen-devel@xxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Juergen Gross <JGross@xxxxxxxx>, Janakarajan Natarajan <Janakarajan.Natarajan@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 19 Oct 2018 17:57:27 +0100
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- Delivery-date: Fri, 19 Oct 2018 16:57:34 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
Hello,
I noticed this most recently in the AVIC series from Janakarajan. The
global svm_avic boolean was left off-by-default because it doesn't work
with nested virt yet. The code in question was actually inherited from
the VT-x side, and the general problem is systemic with how Xen has been
developed in the past.
Nested virt is the easiest way to spot why this is an issue. How to
correctly inject an interrupt into a VM depends on the current
configuration in the VMCB/VMCS, and in the case of nested virt, this is
chosen (at least in part) by the L1 hypervisor. Therefore, a global
boolean isn't correct, because it cannot account for the fact that two
VMCBs/VMCSs might be configured differently.
Other settings I've noticed recently are hap/iommu PT sharing, and
unrestricted_guest, both of which would be more convenient for
development if they were configurable per domain rather than requiring a
host reboot to change.
In practice, having fine grain control of all the features like would be
excellent for testing purposes, because it allows you to boot two
otherwise-identical VMs with one configuration difference between them.
In the spirit of the already in progress domaincreate work, options like
these should be selectable at domain creation time, and immutable
thereafter.
That said, there is a plethora of tweakables, and I'm not sure how best
to expose them. While most (all?) of these options are inherently
supported (as playing with them simulates what Xen would chose on
different hardware), I expect there will be ample opportunity for people
to break their systems if they tweak too much.
Is there liable to be any provision in xl/libxl to have "unstable"
configuration, which is easily identified as "may stop working / cease
to exist / become invalid at any point in the future?"
Alternatively, are there any other suggestions for alternative mechanisms?
Thanks,
~Andrew
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