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

[Andi Kleen]

On Wed, Aug 22, 2007 at 09:48:25AM -0700, Zachary Amsden wrote:
> Andi Kleen wrote:
> >On Tue, Aug 21, 2007 at 10:23:14PM -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.  
> >>    
> >
> >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

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