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

[Ben Catterall <Ben.Catterall@citrix.com>]

On 12/08/15 10:50, Jan Beulich wrote:
>>>> On 06.08.15 at 18:45, <Ben.Catterall@citrix.com> 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
>

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.

Thanks,
Ben