Re: [Xen-devel] [RFC 0/4] HVM x86 enhancements to run Xen deprivileged mode operations

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.08.15 at 18:45, <Ben.Catterall@xxxxxxxxxx> wrote:
> Performance testing
> -------------------
> Performance testing indicates that the overhead for this deprivileged mode is
> approximately 25%. This overhead is the cost of moving into deprivileged mode
> and then fully back out of deprivileged mode.
> 
> I performed 100000 writes to a single I/O port on an Intel 2.2GHz Xeon
> E5-2407 0 processor. This was done from a python script within the HVM guest
> using time.time() and running Debian Jessie. Each write was trapped to cause a
> vmexit and the time for each write was calculated. These experiments were
> repeated. Note that only the host and this HVM guest were running (both Debian
> Jessie) during the experiments.

I'm not sure I/O port writes (the more without saying to which port)
are the best suited item for evaluating the overhead: Ideally you
would pick an operation that is comparably fast (I/O port accesses
generally aren't if the net effect is that the emulator accesses a
physical port). But it certainly serves as a first indication (not too
bad, but large enough that I wouldn't be convinced this is a good
idea).

Jan


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