On Wed, Aug 22, 2007 at 09:48:25AM -0700, Zachary Amsden wrote:

Andi Kleen wrote:

quoted

On Tue, Aug 21, 2007 at 10:23:14PM -0700, Zachary Amsden wrote:
 

quoted

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.  
   

Is that really that expensive? Hard to imagine.
 

You have an expensive (16x cost of hypercall on some processors) 

Where is the difference comming from? Are you using SYSENTER
for the hypercall?  I can't really see you using SYSENTER,
because how would you do system calls then? I bet system calls
are more frequent than in/out, so if you have decide between the
two using them for syscalls is likely faster.

For an int XYZ gate i wouldn't expect that much difference to
a #GP fault.

Also I fail to see the fundamental speed difference between

mov index,register
int 0x...
...
switch (register) 
case xxxx: do emulation

versus

out ...
#gp
-> switch (*eip) {
case 0xee:  /* etc. */ 
	do emulation

privilege transition, you have to decode the instruction, then you have 

out is usually a single byte. Shouldn't be very expensive
to decode. In fact it should be roughly equivalent to your
hypercall multiplex.

to verify protection in the page tables mapping the page allows 
execution (P, !NX, and U/S check).  This is a lot more expensive than a 

When the page is not executable or not present you get #PF not #GP. 
So the hardware already checks that.

The only case where you would need to check yourself is if you emulate
NX on non NX capable hardware, but I can't see you doing that.

There are 24 different possible I/O operations; sometimes with a port 
encoded in the instruction, sometimes with input in the DX register, 
sometimes with a rep prefix, and for 3 different operand sizes.

Most of this is a single byte which is the same as the hypercall
demux. Essentially a table lookup if you use the obvious switch()

-Andi