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Re: [Xen-devel] IOMMU page-tables
- To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>, "xen-devel (xen-devel@xxxxxxxxxxxxxxxxxxxx)" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 18 Jul 2019 11:13:34 +0100
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- Delivery-date: Thu, 18 Jul 2019 10:13:43 +0000
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On 18/07/2019 10:49, Paul Durrant wrote:
> Hi,
>
> Before I get too far into this I want to get some opinions from the wider
> community...
>
> At the moment when the first PCI device is assigned to a domain (i.e.
> passed through) this will trigger construction of IOMMU page tables for that
> domain. Similarly when the last PCI device is de-assigned the tables are torn
> down again. Both of these operations can be quite expensive if the domain is
> a large amount for RAM, and if the IOMMU does not support sharing the second
> level CPU page tables (or the domain is PV). Moreover, the guest could
> simultaneously be ballooning or doing other operations that affect its page
> types and so the process of building the tables has to take into account such
> races.
> If we were instead to say that giving a domain IOMMU page tables is an
> option that needs to be selected at domain create time then it makes the code
> significantly simpler and there is no need to deal with page type
> change/ballooning races any more. I hacked together a test patch and it gets
> rid of roughly 200 lines of code (and there may be more that I missed).
> Does anyone think that deciding whether a domain should get IOMMU page
> tables is not a reasonable thing to have to do at create time?
To be absolutely clear here, I don't see any evidence for the current
behaviour to be a commonly used feature in practice.
If anyone has a counterexample, please speak up urgently.
Furthermore, the complexity and (best-we-can-manage-security-wise)
livelocks in the hypervisor/toolstack is justification alone to
seriously consider dropping it moving forwards. The complexity in
particular is an impediment to several aspects of IOMMU development work
which are in progress.
In particular, I do not see it as an unreasonable burden for
admins/guests to specify whether they want an IOMMU at domain creation
time. I expect that the overwhelming majority of cases already know at
creation time which PCI devices want assigning.
As far as I'm concerned, this feature/corner case wants ripping out.
Whatever the original intention, hindsight has demonstrated that it
really isn't clever idea.
~Andrew
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