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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 02/12/2014 12:53 AM, Zhang, Yang Z wrote:
> George Dunlap wrote on 2014-02-11:
> > On 02/11/2014 12:57 PM, Jan Beulich wrote:
> > > > > > On 11.02.14 at 12:55, Tim Deegan <tim@xxxxxxx> wrote:
> > > > At 10:59 +0000 on 11 Feb (1392112778), George Dunlap wrote:
> > > > > What I'm missing here is what you think a proper solution is.
> > > > A _proper_ solution would be for the IOMMU h/w to allow restartable
> > > > faults, so that we can do all the usual fault-driven virtual memory
> > > > operations with DMA. :)  In the meantime...
> > > Or maintaining the A/D bits for IOMMU side accesses too.
> > >
> > > > >    It seems we have:
> > > > > A. Share EPT/IOMMU tables, only do log-dirty tracking on the
> > > > > buffer being tracked, and hope the guest doesn't DMA into video
> > > > > ram; DMA causes IOMMU fault. (This really shouldn't crash the host
> > > > > under normal circumstances; if it does it's a hardware bug.)
> > > > Note "hope" and "shouldn't" there. :)
> > > >
> > > > > B. Never share EPT/IOMMU tables, and hope the guest doesn't DMA
> > > > > into video ram.  DMA causes missed update to dirty bitmap, which
> > > > > will hopefully just cause screen corruption.
> > > > Yep.  At a cost of about 0.2% in space and some extra bookkeeping
> > > > (for VMs that actually have devices passed through to them).
> > > > The extra bookkeeping could be expensive in some cases, but
> > > > basically all of those cases are already incompatible with IOMMU.
> > > >
> > > > > C. Do buffer scanning rather than dirty vram tracking (SLOW) D.
> > > > > Don't allow both a virtual video card and pass-through
> > > > E. Share EPT and IOMMU tables until someone turns on log-dirty mode
> > > > and then split them out.  That one
> > > Wouldn't that be problematic in terms of memory being available,
> > > namely when using ballooning in Dom0?
> > >
> > > > > Given that most operating systems will probably *not* DMA into
> > > > > video ram, and that an IOMMU fault isn't *supposed* to be able to
> > > > > crash the host, 'A' seems like the most reasonable option to me.
> > > > Meh, OK.  I prefer 'B' but 'A' is better than nothing, I guess, and
> > > > seems to have most support from other people.  On that basis this
> > > > patch can have my Ack.
> > > I too would consider B better than A.
> > 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.
 -George


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