[Qemu-devel] Re: [PATCH 0/9] S390x KVM support

[Avi Kivity]

On 10/22/2009 12:22 PM, Carsten Otte wrote:
> Avi Kivity wrote:
> > Right, but why?  x86 qemu doesn't care about either pc or eflags 
> > (with in-kernel irqchip, which s390 essentially is).
> For different reasons. Most prominent for setting the condition code,
> which is a sideband result of most instructions that indicates whether or
> not the instruction actually worked - similar to the exception model in
> high level programming languages.

Ok.  Thanks for the explanation.

On x86 we avoid emulating instructions in userspace.  Instead the kernel 
requests userspace to do something (triggered by the instruction), and 
the kernel does anything which might be implied by the instruction (like 
copying the result into a register, or updating pc).

An example is port I/O.  instead of userspace reading %edx to query the 
port number and setting %eax to indicate the result, userspace reads a 
port number struct field and writes an I/O result struct field.  Only 
the kernel accesses registers.

I don't know whether that model makes sense or not for s390, but please 
consider it.

--
error compiling committee.c: too many arguments to function