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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.
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.

Jan


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