Re: [PATCH] Add I/O hypercalls for i386 paravirt

Benjamin Herrenschmidt
2007-08-28T06:50:16Z

On Tue, 2007-08-21 at 22:23 -0700, Zachary Amsden wrote:
> In general, I/O in a virtual guest is subject to performance problems.
> The I/O can not be completed physically, but must be virtualized. This
> means trapping and decoding port I/O instructions from the guest OS.
> Not only is the trap for a #GP heavyweight, both in the processor and
> the hypervisor (which usually has a complex #GP path), but this forces
> the hypervisor to decode the individual instruction which has faulted.
> Worse, even with hardware assist such as VT, the exit reason alone is
> not sufficient to determine the true nature of the faulting instruction,
> requiring a complex and costly instruction decode and simulation.

.../...

How about userland ? Things like X do IO's typically... You still need
to trap/emulate for these no ?

Cheers,
Ben.