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Re: [Qemu-devel] [PATCH qemu v7 06/14] spapr_iommu: Introduce "enabled"

From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] [PATCH qemu v7 06/14] spapr_iommu: Introduce "enabled" state for TCE table
Date: Wed, 27 May 2015 00:17:42 +1000
User-agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 05/27/2015 12:03 AM, Paolo Bonzini wrote:

On 26/05/2015 16:00, Alexey Kardashevskiy wrote:
On 05/26/2015 11:48 PM, Paolo Bonzini wrote:

On 26/05/2015 15:42, Alexey Kardashevskiy wrote:

The next patch of this patchset changes:
      memory_region_add_subregion(&root, &iommu)

      memory_region_del_subregion(&root, &iommu)

These spapr_tce_xxx are called by request from the guest. &root is a
container and exists as long as sPAPRTCETable exists.

Where do I get a leaking child property here?

When you unref iommu and not unparent it.  The next
memory_region_init_iommu creates a second child property, and the first
is gone.

But when do I get this child property? In memory_region_add_subregion()?
And memory_region_del_subregion() does not do the opposite thing

In memory_region_init_iommu.

Ah. So I need at least s/object_unref/object_unparent/ in my current code, right?

What is different between the various IOMMU regions, so that you cannot
create just one?

There are two DMA windows on the same PCI bus (in hardware too), at
different offset and with a different page size.

Why do you need different regions?  Why can't you have always the same
IOMMU regions, and either:

They may change a size. These are dynamic DMA windows, guest may remove all and create randomly. Each region is backed by a separate TCE table with different page size.

1) create/destroy an alias to that region

How does this change things compared to iommus in regard to parenting?

2) change the behavior of the translation function, while keeping a
single region?

Have one sPAPRTCETable object with 0, 1 or 2 (and potentially more) actual TCE tables? I can do that too but I thought subregions are just natural for that. I even wanted to create sPAPRTCETable' dynamically but this would break migration (because we cannot start QEMU with an additional sPAPRTCETable if it exists in the source which is not always the case).

Ok. I'll redo this thing again and try using less QOM objects...


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