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Re: [Xen-devel] [PATCH v3] x86/mm: don't needlessly veto migration
- To: Jürgen Groß <jgross@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- From: George Dunlap <george.dunlap@xxxxxxxxxx>
- Date: Wed, 9 Oct 2019 15:14:48 +0100
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- Cc: Wei Liu <wl@xxxxxxx>, Paul Durrant <paul@xxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 09 Oct 2019 14:15:06 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 10/9/19 11:23 AM, Jürgen Groß wrote:
>> Regardless of the merits of the change Andy wants to see, it's not a one
>> that should be made during a feature freeze.
>
> Indeed. So either we take this patch or we have to revert the patch(es)
> introducing the regression.
Actually, just chatting with Ian -- the worse issue ATM, AFAICT, is that
the IOMMU is enabled for a guest which has neither asked for PCI
devices, nor explicitly enabled it; and he's currently working on a fix
for that. Once that issue is fixed, then osstest should become
unblocked again.
It is, arguably, not ideal to refuse to migrate a VM with IOMMU enabled
but no devices attached; but if it only affected guests who had
specifically requested the IOMMU be enabled, that wouldn't be so
terrible. (And in fact it has highlighted the other, more important issue.)
In summary, this patch is not strictly needed to get a push to osstest.
That said, the behavior in 4.12 was, as far as I can tell:
1. If a guest had never had a PCI device assigned, Xen will allow
logdirty to be enabled.
2. If a guest has a PCI device assigned, Xen will not allow logdirty to
be enabled (blocking migration).
3. If a guest had a PCI device assigned in the past but does not have
one now, Xen will also not allow logdirty to be enabled (blocking
migration).
At the moment, once Ian fixes the default, the behavior will be:
1 If a guest never had PCI device assigned, and
a. the IOMMU was not explicitly enabled, Xen will allow logdirty to be
enabled
b. the IOMMU was explicitly enabled, xen will not allow logdirty
1a, 2, and 3 are the same, and 1b didn't exist in 4.12, so arguably
there's no regression.
This patch changes things so that migration will work in the 1b and 3
cases (if I'm reading it right). This is arguably better, but also
arguably an unnecessary change post-freeze.
And of course there's the option Andy is proposing, of having Xen allow
logdirty to be enabled in all cases, and having the toolstack keep track
of whether there are devices assigned and refuse migration if so; that's
a technical change which should be avoided post-freeze.
-George
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