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

From: Peter Xu
Subject: Re: [Qemu-devel] [PATCH RFC v3 11/14] intel_iommu: provide its own replay() callback
Date: Mon, 16 Jan 2017 15:59:47 +0800
User-agent: Mutt/1.5.24 (2015-08-30)

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

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