[Xen-devel] Re: [RFC]vmx: Enable direct hardware n/p fault injection

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

Hi,

At 15:24 +0800 on 04 Feb (1202138657), Tian, Kevin wrote:
> VT-x supports direct reflect for some type of page faults,
> without triggering VM-exit. At least we can utilize this
> feature to accelerate guest n/p fault. The key thing is
> to only clear shadow present bit if guest entry really does.
> All other cases are considered out-of-sync (oos) with guest
> which thus are all set with a special magic with reserved
> bit set.
> 
> Effectively this just replaces fast n/p injection at same
> places, and is also controlled by SHOPT_FAST_FAULT_PATH.
> Thus the gain of this change is straightforward to release
> vmexit/vmentry overhead for existing fast n/p path, though
> it's already 'fast', with negligible overhead on other
> paths when checking shadow flags.
> 
> Our test shows no obvious increase for some benchmark
> scores, then with 0.9% improvement for PAE KB, and 0.7%
> for PAE windows sysbench. It depends on frequency of
> guest n/p fault.

It's a clever idea, though it does add yet one more level of
brain-twisting to the shadow code. :)  I'm hoping to measure what this
looks like on AMD machines this week; the extra code on hot paths will
potentially make a measurable slowdown there.

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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