From mboxrd@z Thu Jan  1 00:00:00 1970
From: Carsten Otte <cotte@de.ibm.com>
Subject: Re: [patch 07/11] [PATCH] kvm-s390-ucontrol: interface to inject
 faults on a vcpu page table
Date: Wed, 04 Jan 2012 17:34:28 +0100
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References: <20120104092519.060746143@de.ibm.com> <20120104093602.530415566@de.ibm.com> <4F0467BE.8010908@redhat.com> <4F0479C7.4060500@de.ibm.com>
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Cc: Marcelo Tossati <mtosatti@redhat.com>, borntrae@linux.vnet.ibm.com,
	heicars2@linux.vnet.ibm.com, mschwid2@linux.vnet.ibm.com,
	huckc@linux.vnet.ibm.com, KVM <kvm@vger.kernel.org>,
	Joachim von Buttlar <joachim_von_buttlar@de.ibm.com>,
	Jens Freimann <jfrei@de.ibm.com>, agraf@suse.de
To: Avi Kivity <avi@redhat.com>
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Avi wrote:
> There is an alternative, if you can recognize this condition exactly
> from the hardware fault, you can fault the lowcore yourself and retry.
> This eliminates a user interface. Is this workable?
I've read your comment again, and understood it this time. Trouble is
that the kernel cannot handle the situation either: userspace may need
to malloc some memory and call KVM_S390_UCAS_MAP prior to resolving it.
One could avoid the user interface by partially handling it in-kernel
and partially handling it in userspace without handshaking:
- user calls KVM_RUN
- SIE validity intercept
- kernel tries to gmap_fault and recognizes -EFAULT
- kernel returns validity intercept to user
- user does KVM_S390_UCAS_MAP
- user calls KVM_RUN
- SIE validity intercept
- kernel tries to gmap_fault and succeeds
- SIE runs ok

I guess I prefer to do processing of one operation in one place, and
thus I prefer the user interface over this. But yes, it'd be workable
without this interface. Is this what you want?