Re: [Xen-devel] [PATCH] Don't track all memory when enabling log dirty to track vram

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

title 38 Implement VT-d large pages so we can avoid sharing between EPT 
and IOMMU
owner it Yang Z Zhang <yang.z.zhang@xxxxxxxxx>
thanks

On 02/13/2014 04:20 PM, Tim Deegan wrote:
> At 15:55 +0000 on 13 Feb (1392303343), Jan Beulich wrote:
> > > > > On 13.02.14 at 16:46, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
> > > On 02/12/2014 12:53 AM, Zhang, Yang Z wrote:
> > > > George Dunlap wrote on 2014-02-11:
> > > > > I think I got a bit distracted with the "A isn't really so bad" thing.
> > > > > Actually, if the overhead of not sharing tables isn't very high, then
> > > > > B isn't such a bad option.  In fact, B is what I expected Yang to
> > > > > submit when he originally described the problem.
> > > > Actually, the first solution came to my mind is B. Then I realized that even
> > > chose B, we still cannot track the memory updating from DMA(even with A/D
> > > bit, it still a problem). Also, considering the current usage case of log
> > > dirty in Xen(only vram tracking has problem), I though A is better.:
> > > Hypervisor only need to track the vram change. If a malicious guest try to
> > > DMA to vram range, it only crashed himself (This should be reasonable).
> > > > > I was going to say, from a release perspective, B is probably the
> > > > > safest option for now.  But on the other hand, if we've been testing
> > > > > sharing all this time, maybe switching back over to non-sharing whole-hog has
> > > the higher risk?
> > > > Another problem with B is that current VT-d large paging supporting relies on
> > > the sharing EPT and VT-d page table. This means if we choose B, then we need
> > > to re-enable VT-d large page. This would be a huge performance impaction for
> > > Xen 4.4 on using VT-d solution.
> > >
> > > OK -- if that's the case, then it definitely tips the balance back to
> > > A.  Unless Tim or Jan disagrees, can one of you two check it in?
> > >
> > > Don't rush your judgement; but it would be nice to have this in before
> > > RC4, which would mean checking it in today preferrably, or early
> > > tomorrow at the latest.
> > That would be Tim then, as he would have to approve of it anyway.
> Done.
>
> > I should also say that while I certainly understand the argumentation
> > above, I would still want to go this route only with the promise that
> > B is going to be worked on reasonably soon after the release, ideally
> > with the goal of backporting the changes for 4.4.1.
> Agreed.
OK -- I've retitled the bug and am going to leave it open.

 -George

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