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

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.

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?

Anyway, both are at least probably equal risk-wise. How easy is it to implement?


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