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Date: Thu, 22 Oct 2009 12:43:54 +0200
From: Carsten Otte <carsteno@de.ibm.com>
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Subject: [Qemu-devel] Re: [PATCH 0/9] S390x KVM support
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To: Avi Kivity <avi@redhat.com>
Cc: Carsten Otte <cotte@de.ibm.com>, carsteno@linux.vnet.ibm.com, KVM list <kvm@vger.kernel.org>, Alexander Graf <agraf@suse.de>, qemu-devel <qemu-devel@nongnu.org>, hare@suse.de

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.