Re: [virtio-comment] [PATCH V2 2/2] virtio: introduce STOP status bit

From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>,
	Eugenio Perez Martin <eperezma@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	virtio-comment@lists.oasis-open.org,
	Virtio-Dev <virtio-dev@lists.oasis-open.org>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Max Gurtovoy <mgurtovoy@nvidia.com>,
	Cornelia Huck <cohuck@redhat.com>, Oren Duer <oren@nvidia.com>,
	Shahaf Shuler <shahafs@nvidia.com>,
	Parav Pandit <parav@nvidia.com>, Bodong Wang <bodong@nvidia.com>,
	Alexander Mikheev <amikheev@nvidia.com>,
	Halil Pasic <pasic@linux.ibm.com>
Subject: Re: [virtio-comment] [PATCH V2 2/2] virtio: introduce STOP status bit
Date: Mon, 12 Jul 2021 12:00:39 +0800	[thread overview]
Message-ID: <778ffa63-c0ca-2faa-32da-3c5e71dd1dfe@redhat.com> (raw)
In-Reply-To: <20210710163326-mutt-send-email-mst@kernel.org>

在 2021/7/11 上午4:36, Michael S. Tsirkin 写道:
> On Fri, Jul 09, 2021 at 07:23:33PM +0200, Eugenio Perez Martin wrote:
>>>>    If I understand correctly, this is all
>>>> driven from the driver inside the guest, so for this to work
>>>> the guest must be running and already have initialised the driver.
>>>
>>> Yes.
>>>
>> As I see it, the feature can be driven entirely by the VMM as long as
>> it intercept the relevant configuration space (PCI, MMIO, etc) from
>> guest's reads and writes, and present it as coherent and transparent
>> for the guest. Some use cases I can imagine with a physical device (or
>> vp_vpda device) with VIRTIO_F_STOP:
>>
>> 1) The VMM chooses not to pass the feature flag. The guest cannot stop
>> the device, so any write to this flag is an error/undefined.
>> 2) The VMM passes the flag to the guest. The guest can stop the device.
>> 2.1) The VMM stops the device to perform a live migration, and the
>> guest does not write to STOP in any moment of the LM. It resets the
>> destination device with the state, and then initializes the device.
>> 2.2) The guest stops the device and, when STOP(32) is set, the source
>> VMM migrates the device status. The destination VMM realizes the bit,
>> so it sets the bit in the destination too after device initialization.
>> 2.3) The device is not initialized by the guest so it doesn't matter
>> what bit has the HW, but the VM can be migrated.
>>
>> Am I missing something?
>>
>> Thanks!
> It's doable like this. It's all a lot of hoops to jump through though.
> It's also not easy for devices to implement.

It just requires a new status bit. Anything that makes you think it's 
hard to implement?

E.g for networking device, it should be sufficient to use this bit + the 
virtqueue state.

> Why don't we design the feature in a way that is useable by VMMs
> and implementable by devices in a simple way?

It use the common technology like register shadowing without any further 
stuffs.

Or do you have any other ideas?

(I think we all know migration will be very hard if we simply pass 
through those state registers).

Thanks

>