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Re: [Qemu-devel] [PATCH v7 0/5] IOMMU: intel_iommu support map and unmap

From: Peter Xu
Subject: Re: [Qemu-devel] [PATCH v7 0/5] IOMMU: intel_iommu support map and unmap notifications
Date: Wed, 30 Nov 2016 17:23:59 +0800
User-agent: Mutt/1.5.24 (2015-08-30)

On Mon, Nov 28, 2016 at 05:51:50PM +0200, Aviv B.D wrote:
> * intel_iommu's replay op is not implemented yet (May come in different patch 
>   set).
>   The replay function is required for hotplug vfio device and to move devices 
>   between existing domains.

I am thinking about this replay thing recently and now I start to
doubt whether the whole vt-d vIOMMU framework suites this...

Generally speaking, current work is throwing away the IOMMU "domain"
layer here. We maintain the mapping only per device, and we don't care
too much about which domain it belongs. This seems problematic.

A simplest wrong case for this is (let's assume cache-mode is
enabled): if we have two assigned devices A and B, both belong to the
same domain 1. Meanwhile, in domain 1 assume we have one mapping which
is the first page (iova range 0-0xfff). Then, if guest wants to
invalidate the page, it'll notify VT-d vIOMMU with an invalidation
message. If we do this invalidation per-device, we'll need to UNMAP
the region twice - once for A, once for B (if we have more devices, we
will unmap more times), and we can never know we have done duplicated
work since we don't keep domain info, so we don't know they are using
the same address space. The first unmap will work, and then we'll
possibly get some errors on the rest of dma unmap failures.

Looks like we just cannot live without knowing this domain layer.
Because the address space is binded to the domain. If we want to sync
the address space (here to setup a correct shadow page table), we need
to do it per-domain.

What I can think of as a solution is that we introduce this "domain"
layer - like a memory region per domain. When invalidation happens,
it's per-domain, not per-device any more (actually I guess that's what
current vt-d iommu driver in kernel is doing, we just ignored it - we
fetch the devices that matches the domain ID). We can/need to maintain
something different, like sid <-> domain mappings (we can do this as
long as we are notified when context entries changed), per-domain
mappings (just like per-device mappings that we are trying to build in
this series, but what we really need is IMHO per domain one), etc.
When device switches domain, we switch the IOMMU memory region

Does this make any sense? Comments are greatly welcomed (especially
from AlexW and DavidG).


-- peterx

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