On Thu, 2010-03-11 at 17:17 +0800, Sheng Yang wrote:
> On Thursday 11 March 2010 15:50:54 Avi Kivity wrote:
> > On 03/11/2010 09:46 AM, Sheng Yang wrote:
> > > On Thursday 11 March 2010 15:36:01 Avi Kivity wrote:
> > >> On 03/11/2010 09:20 AM, Sheng Yang wrote:
> > >>> Currently we can only get the cpu_stat of whole guest as one. This
> > >>> patch enhanced cpu_stat with more detail, has guest_system and
> > >>> guest_user cpu time statistics with a little overhead.
> > >>>
> > >>> Signed-off-by: Sheng Yang<sheng@xxxxxxxxxxxxxxx>
> > >>> ---
> > >>>
> > >>> This draft patch based on KVM upstream to show the idea. I would split
> > >>> it into more kernel friendly version later.
> > >>>
> > >>> The overhead is, the cost of get_cpl() after each exit from guest.
> > >>
> > >> This can be very expensive in the nested virtualization case, so I
> > >> wouldn't like this to be in normal paths. I think detailed profiling
> > >> like that can be left to 'perf kvm', which only has overhead if enabled
> > >> at runtime.
> > >
> > > Yes, that's my concern too(though nested vmcs/vmcb read already too
> > > expensive, they should be optimized...).
> >
> > Any ideas on how to do that? Perhaps use paravirt_ops to covert the
> > vmread into a memory read? We store the vmwrites in the vmcs anyway.
>
> 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.
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.
Thanks,
Qing
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