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Re: [Qemu-devel] [PATCH RFC v3 11/14] intel_iommu: provide its own repla

From: Jason Wang
Subject: Re: [Qemu-devel] [PATCH RFC v3 11/14] intel_iommu: provide its own replay() callback
Date: Mon, 16 Jan 2017 15:47:08 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

On 2017年01月16日 15:31, Peter Xu wrote:
On Fri, Jan 13, 2017 at 05:26:06PM +0800, Jason Wang wrote:

On 2017年01月13日 11:06, Peter Xu wrote:
The default replay() don't work for VT-d since vt-d will have a huge
default memory region which covers address range 0-(2^64-1). This will
normally bring a dead loop when guest starts.
I think it just takes too much time instead of dead loop?
Hmm, I can touch the commit message above to make it more precise.

The solution is simple - we don't walk over all the regions. Instead, we
jump over the regions when we found that the page directories are empty.
It'll greatly reduce the time to walk the whole region.
Yes, the problem is memory_region_is_iommu_reply() not smart because:

- It doesn't understand large page
- try go over all possible iova

So I'm thinking to introduce something like iommu_ops->iova_iterate() which

1) accept an start iova and return the next exist map
2) understand large page
3) skip unmapped iova
Though I haven't tested with huge pages yet, but this patch should
both solve above issue? I don't know whether you went over the page
walk logic - it should both support huge page, and it will skip
unmapped iova range (at least that's my goal to have this patch). In
that case, looks like this patch is solving the same problem? :)
(though without introducing iova_iterate() interface)

Please correct me if I misunderstood it.

Kind of :) I'm fine with this patch, but just want:

- reuse most of the codes in the patch
- current memory_region_iommu_replay() logic

So what I'm suggesting is a just slight change of API which can let caller decide it need to do with each range of iova. So it could be reused for other things except for replaying.

But if you like to keep this patch as is, I don't object it.

To achieve this, we provided a page walk helper to do that, invoking
corresponding hook function when we found an page we are interested in.
vtd_page_walk_level() is the core logic for the page walking. It's
interface is designed to suite further use case, e.g., to invalidate a
range of addresses.

Signed-off-by: Peter Xu<address@hidden>
For intel iommu, since we intercept all map and unmap, a more tricky ieda is
to we can record the mappings internally in something like a rbtree which
could be iterated during replay. This saves possible guest io page table
traversal, but drawback is it may not survive from OOM attacker.
I think the problem is that we need this rbtree per guest-iommu-domain
(because mapping can be different per domain). In that case, I failed
to understand how the tree can help here. :(

Right, I see.



-- peterx

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