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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 16:03:22 +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 15:59, Peter Xu wrote:
On Mon, Jan 16, 2017 at 03:47:08PM +0800, Jason Wang wrote:

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.
I see. Then I can understand your mean here. I had the same thought
before, that's why I exposed the vtd_page_walk with a hook. If you
check the page_walk function comment:

  * vtd_page_walk - walk specific IOVA range, and call the hook
  * @ce: context entry to walk upon
  * @start: IOVA address to start the walk
  * @end: IOVA range end address (start <= addr < end)
  * @hook_fn: the hook that to be called for each detected area
  * @private: private data for the hook function

So I didn't implement the notification in page_walk at all - but in
the hook_fn. If any caller that is interested in doing something else
rather than the notification, we can just simply export the page walk
interface and provide his/her own "hook_fn", then it'll be triggered
for each valid page (no matter a huge/small one).

If we can have a more general interface in the future - no matter
whether we call it iova_iterate() or something else (I'll prefer the
hooker way to do it, so maybe a common page walker with a hook
function), we can do it simply (at least for Intel platform) based on
this vtd_page_walk thing.


-- peterx

Yes but the problem is hook_fn is only visible inside intel iommu code.


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