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

[Avi Kivity]

On 10/22/2009 12:43 PM, Carsten Otte wrote:
> Avi Kivity wrote:
> > 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.
> We do the same for many instructions (arch/s390/kvm/instruction.c). 
> User exits
> are only performed for isntructions that cannot be handled in kernel. 
> Also, we do exit with requests to userspace, see the s390_reset exit 
> reason. I think in this regard, our implementation is very similar to 
> x86. Btw: this is something I did copycat from your implementation on 
> integration into kvm. The original zlive code did handle all 
> instructions in userland.

So why not do it for this instruction as well?  Instead of updating the 
psw, return a success/error code and let the kernel update psw.

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