netdev - Re: [PATCH 0/2] vhot-net: Use kvm_memslots instead of vhost

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Date:	Fri, 16 Dec 2011 16:18:45 +0800
From:	Zang Hongyong <zanghongyong@...wei.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	linux-kernel@...r.kernel.org, mst@...hat.com, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	xiaowei.yang@...wei.com, hanweidong@...wei.com,
	wusongwei@...wei.com
Subject: Re: [PATCH 0/2] vhot-net: Use kvm_memslots instead of vhost_memory to
 translate GPA to HVA

于 2011/12/16,星期五 15:59, Sasha Levin 写道:
> On Fri, 2011-12-16 at 15:40 +0800, Zang Hongyong wrote:
>> 于 2011/12/16,星期五 15:05, Sasha Levin 写道:
>>> On Fri, 2011-12-16 at 13:32 +0800, zanghongyong@...wei.com wrote:
>>>> From: Hongyong Zang<zanghongyong@...wei.com>
>>>>
>>>> Vhost-net uses its own vhost_memory, which results from user space (qemu) info,
>>>> to translate GPA to HVA. Since kernel's kvm structure already maintains the
>>>> address relationship in its member *kvm_memslots*, these patches use kernel's
>>>> kvm_memslots directly without the need of initialization and maintenance of
>>>> vhost_memory.
>>> Conceptually, vhost isn't aware of KVM - it's just a driver which moves
>>> data from vq to a tap device and back. You can't simply add KVM specific
>>> code into vhost.
>>>
>>> Whats the performance benefit?
>>>
>> But vhost-net is only used in virtualization situation. vhost_memory is 
>> maintained
>> by user space qemu.
>> In this way, the memory relationship can be accquired from kernel 
>> without the
>> need of maintainence of vhost_memory from qemu.
> You can't assume that vhost-* is used only along with qemu/kvm. Just as
> virtio has more uses than just virtualization (heres one:
> https://lkml.org/lkml/2011/10/25/139 ), there are more uses for vhost as
> well.
>
> There has been a great deal of effort to keep vhost and kvm untangled.
> One example is the memory translation it has to do, another one is the
> eventfd/irqfd thing it does just so it could signal an IRQ in the guest
> instead of accessing the guest directly.
>
> If you do see a great performance increase when tying vhost and KVM
> together, it may be worth it to create some sort of an in-kernel
> vhost-kvm bridging thing, but if the performance isn't noticeable we're
> better off just leaving it as is and keeping the vhost code general.
>
Thanks for your explanation.
Since memory layout is seldom changed after guest boots, the situation
manily occurs during initialization. There's no need for vhost-kvm
bridge now.

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