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

On 12/08/15 10:50, Jan Beulich wrote:
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


Thanks for this Jan: I overlooked this.

Retested with the port (0x1000) i/o emulation operation replaced with a nop to get the pure overhead. With the operation takes 5.2e-6s and without takes 1.7e-6s. So the difference is 3.5e-6. That's about 308% overhead on a nop. So, I'm guessing that whether this is used or not will depend on the overhead and frequency of the operation it will be handling.


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