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Re: [Xen-devel] [PATCH v3] x86/mm: don't needlessly veto migration
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: George Dunlap <george.dunlap@xxxxxxxxxx>
- Date: Wed, 9 Oct 2019 15:31:18 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Wei Liu <wl@xxxxxxx>, Paul Durrant <paul@xxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 09 Oct 2019 14:31:26 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 10/9/19 3:28 PM, Jan Beulich wrote:
> On 09.10.2019 16:14, George Dunlap wrote:
>> 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).
>
> No - the connection previously was to whether IOMMU page tables
> had been set up; these page tables would have been torn down
> upon de-assignment of the last device, allowing migration again.
> People actually use this behavior afaik, using a bond of a
> passed through SR-IOV NIC and netfront provided device. To
> migrate the VM, the SR-IOV NIC is taken away without the domain
> losing network access, and a new one might then be assigned
> again after migration.
>
> The "IOMMU page tables set up" property was previously identical
> to "has devices assigned", i.e. even before we could have used
> has_arch_pdevs() instead of is_iommu_enabled().
I didn't realize that. So yes, this patch fixes a regression over and
above the toolstack fix Ian is working on.
-George
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