On 03/12/2010 10:53 AM, Qing He wrote:
When Qing(CCed) was working on nested VMX in the past, he found PV
vmread/vmwrite indeed works well(it would write to the virtual vmcs so vmwrite
can also benefit). Though compared to old machine(one our internal patch shows
improve more than 5%), NHM get less benefit due to the reduced vmexit cost.
One of the hurdles to PVize vmread/vmwrite is the fact that the memory
layout of physical vmcs remains unknown. Of course it can use the custom
vmcs layout utilized by nested virtualization, but that looks a little weird,
since different nested virtualization implementation may create different
custom layout.
Note we must use a custom layout and cannot depend on the physical
layout, due to live migration. The layout becomes an ABI.
I once used another approach to partially accelerate the vmread/vmwrite
in nested virtualization case, which also gives good performance gain (around
7% on pre-nehalem, based on this, PV vmread/vmwrite had another 7%). That
is to make a shortcut to handle EXIT_REASON_VM{READ,WRITE}, without
even turning on the IF.
Interesting. That means our exit path is inefficient; it seems to imply
half the time is spent outside the hardware vmexit path.
A quick profile (on non-Nehalem) shows many atomics and calls into the
lapic, as well as update_cr8_intercept which is sometimes unnecessary;
these could easily be optimized.
Definitely optimizing the non-paravirt path is preferred to adding more
paravirtualization.
--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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