Re: [RFC PATCH 2/5] KVM: add KVM_EXIT_MSR exit reason and capability.

From: 'Roman Kagan' <rkagan@virtuozzo.com>
To: Pavel Fedin <p.fedin@samsung.com>
Cc: 'Peter Hornyack' <peterhornyack@google.com>,
	'Paolo Bonzini' <pbonzini@redhat.com>,
	'kvm list' <kvm@vger.kernel.org>,
	'Gleb Natapov' <gleb@kernel.org>,
	'Joerg Roedel' <joro@8bytes.org>,
	'Andrey Smetanin' <asmetanin@virtuozzo.com>,
	"'Denis V. Lunev'" <den@openvz.org>
Subject: Re: [RFC PATCH 2/5] KVM: add KVM_EXIT_MSR exit reason and capability.
Date: Tue, 22 Dec 2015 15:01:07 +0300	[thread overview]
Message-ID: <20151222120106.GJ11808@rkaganb.sw.ru> (raw)
In-Reply-To: <003701d13c89$c2d5e980$4881bc80$@samsung.com>

On Tue, Dec 22, 2015 at 10:24:13AM +0300, Pavel Fedin wrote:
> > +On the return path into kvm, user space should set handled to
> > +KVM_EXIT_MSR_HANDLED if it successfully handled the MSR access. Otherwise,
> > +handled should be set to KVM_EXIT_MSR_UNHANDLED, which will cause a general
> > +protection fault to be injected into the vcpu. If an error occurs during the
> > +return into kvm, the vcpu will not be run and another exit will be generated
> > +with type set to KVM_EXIT_MSR_COMPLETION_FAILED.
> > +
> > +If exit_reason is KVM_EXIT_MSR_AFTER_WRITE, then the vcpu has executed a wrmsr
> > +instruction which is handled by kvm but which user space may need to be
> > +notified about. index and data are set as described above; the value of type
> > +depends on the MSR that was written. handled is ignored on reentry into kvm.
> 
> 1. Is there any real need to distinguish between KVM_EXIT_MSR_WRITE and KVM_EXIT_MSR_AFTER_WRITE ? IMHO from userland's point of view these are the same.

Indeed.  Perhaps the kernel can set .handled to true to let userspace
know it already took care of it, instead of introducing yet another
exit_reason.  The field would need to be marked in/out, then.

Roman.