Re: [Xen-devel] [PATCH RFC v2 0/4] HVM x86 deprivileged mode operations

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.09.15 at 18:01, <Ben.Catterall@xxxxxxxxxx> wrote:
> I performed 100000 writes to a single I/O port on an Intel 2.2GHz Xeon
> E5-2407 0 processor and an AMD Opteron 2376. 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. The 
> port
> operation is bypassed so that no portio is actually performed. Thus, the
> differences in the measurements below can be taken as the pure overhead. 
> These
> experiments were repeated. Note that only the host and this HVM guest were
> running (both Debian Jessie) during the experiments.
> 
> Intel Intel 2.2GHz Xeon E5-2407 0 processor:
> --------------------------------------------
> 1.55e-06 seconds was the average time for performing the write without the
>          deprivileged code running.
> 
> 5.75e-06 seconds was the average time for performing the write with the
>          deprivileged code running.
> 
> So approximately 351% overhead
> 
> AMD Opteron 2376:
> -----------------
> 1.74e-06 seconds was the average time for performing the write without the
>          deprivileged code running.
> 3.10e-06 seconds was the average time for performing the write with an entry 
> and
>          exit from deprvileged mode.
> 
> So approximately 178% overhead.

Just like said for v1: Determining a percentage of overhead is
pretty meaningless when the actual operation (the I/O port
access) can take significantly varying amount of time depending
on which I/O port is being accessed. In particular, considering
the built in devices emulation of which you want to move out,
the majority shouldn't actually be doing any accesses to ports
or MMIO, but just act on RAM. Which hence may take quite a
bit less than the roughly 1.5us you use as the base line, in turn
likely resulting in quite a bit higher relative overhead.

That said - even the 350% you determined above look
prohibitive to me.

Jan


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