Re: [Xen-users] HVM or not HVM?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > There's a nice nutshell explanation at
> > http://virt.kernelnewbies.org/TechOverview. Basically certain operations
> > are *extremely* expensive to virtualization. Para-virtualized kernels
> > simply don't use those operations. (I use the word "simply" in a very
> > genteel sort of way, it's not that simple.)

Paravirt is faster because it avoids trapping and emulating pretty much 
anything, and allows the guest to explicitly co-operate with the 
virtualisation process.  HVM is a bit more heavyweight: it allows you to run 
non-PV guests, but doing this implies you're doing more emulation.

The HVM hardware extensions are not inherently slower than using PV.  But 
emulating a whole machine using them is.  This is partly because emulation of 
some machine features is inefficient, and partly because some aspects of 
virtualisation (e.g. shadow page tables) are not supported as efficiently by 
current generations of HVM hardware as they could be (and will be in future).

The theoretical optimum would be to use PV *and* HVM at once.  i.e. use 
paravirtualisation when it speeds things up or makes them more simple; use 
emulation using HVM hardware when it cuts down the diff from the native 
version of the OS, or when it allows optimisations.

An example would be that VT has acceleration for context switches.  This 
could, in principle, be used to be used to accelerate context switches when 
running PV guests but it currently isn't.  An "ideal" solution would use VT 
to accelerate this when available, even for PV.

The approach of combining elements of hardware-supported virtualisation and 
software-only paravirtualisation is sometimes called "hybrid virtualisation".  
There's a paper on it from this years OLS: 
https://ols2006.108.redhat.com/2007/Reprints/nakajima-Reprint.pdf

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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