RE: [PATCH v1 4/5] kvm/x86: Hyper-V VMBus hypercall userspace exit

From: Pavel Fedin <p.fedin@samsung.com>
To: 'Paolo Bonzini' <pbonzini@redhat.com>,
	'Andrey Smetanin' <asmetanin@virtuozzo.com>,
	kvm@vger.kernel.org
Cc: 'Gleb Natapov' <gleb@kernel.org>,
	'Joerg Roedel' <joro@8bytes.org>,
	"'K. Y. Srinivasan'" <kys@microsoft.com>,
	'Haiyang Zhang' <haiyangz@microsoft.com>,
	'Roman Kagan' <rkagan@virtuozzo.com>,
	"'Denis V. Lunev'" <den@openvz.org>,
	qemu-devel@nongnu.org
Subject: RE: [PATCH v1 4/5] kvm/x86: Hyper-V VMBus hypercall userspace exit
Date: Wed, 20 Jan 2016 17:41:59 +0300	[thread overview]
Message-ID: <005d01d15390$b852b880$28f82980$@samsung.com> (raw)
In-Reply-To: <569F92AC.4030506@redhat.com>

 Hello!

> Because, as the docs say, we don't want to do that.

 Where do they say this? I remember that "we want to use KVM_EXIT_IO or KVM_EXIT_MMIO for handling device I/O". In other words - we
should not introduce anything that requires any other mechanism (e. g. hypercalls) to handle this. And that's all. By the way,
Hyper-V implementation contradicts this rule by itself. And i wonder why we need it at all, but, well, why not. So, Hyper-V violates
this rule only because it was designed by other people under different rules. And we have to follow these rules. So we have to
support hypercalls. And hypercalls cannot map to EXIT_IO or EXIT_MMIO, so...

>  We want to use
> KVM_EXIT_IO or KVM_EXIT_MMIO, with two exceptions: s390 and wherever we
> can't do that for compatibility purposes.  This is the latter.

 Yes, basically this is what i told above...

>  So we
> should not add a new exit (I suggested elsewhere the existing hyper-v
> exit)

 Yes, this would also be more consistent i think, if we think subsystem-centric ("we are implementing Hyper-V") instead of
implementation-centric ("we are implementing hypercalls").

Kind regards,
Pavel Fedin
Senior Engineer
Samsung Electronics Research center Russia