Re: [Qemu-devel] [PATCH v4 for-4.0 0/7] vhost-user-blk: Add support for backend reconnecting

From: Stefan Hajnoczi <stefanha@gmail.com>
To: Yongji Xie <elohimes@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>,
	"Daniel P. Berrangé" <berrange@redhat.com>,
	"Jason Wang" <jasowang@redhat.com>,
	"Coquelin, Maxime" <maxime.coquelin@redhat.com>,
	"Yury Kotov" <yury-kotov@yandex-team.ru>,
	"Евгений Яковлев" <wrfsh@yandex-team.ru>,
	nixun@baidu.com, qemu-devel <qemu-devel@nongnu.org>,
	lilin24@baidu.com, zhangyu31@baidu.com, chaiwen@baidu.com,
	"Xie Yongji" <xieyongji@baidu.com>
Subject: Re: [Qemu-devel] [PATCH v4 for-4.0 0/7] vhost-user-blk: Add support for backend reconnecting
Date: Wed, 16 Jan 2019 14:28:25 +0000	[thread overview]
Message-ID: <20190116142825.GG16898@stefanha-x1.localdomain> (raw)
In-Reply-To: <CAONzpcbomC21=7irxFEYEfA9=EGUUfNUZVNxZXRA0Rwb5MWMMg@mail.gmail.com>

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On Mon, Jan 14, 2019 at 06:55:40PM +0800, Yongji Xie wrote:
> On Mon, 14 Jan 2019 at 18:22, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> >
> > On Sat, Jan 12, 2019 at 12:50:12PM +0800, Yongji Xie wrote:
> > > On Fri, 11 Jan 2019 at 23:53, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > > >
> > > > On Thu, Jan 10, 2019 at 06:59:27PM +0800, Yongji Xie wrote:
> > > > > On Thu, 10 Jan 2019 at 18:25, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > > > > >
> > > > > > On Wed, Jan 09, 2019 at 07:27:21PM +0800, elohimes@gmail.com wrote:
> > > > I'm concerned that this approach to device recovery is invasive and hard
> > > > to test.  Instead I would use VIRTIO's Device Status Field
> > > > DEVICE_NEEDS_RESET bit to tell the guest driver that a reset is
> > > > necessary.  This is more disruptive - drivers either have to resubmit or
> > > > fail I/O with EIO - but it's also simple and more likely to work
> > > > correctly (it only needs to be implemented correctly in the guest
> > > > driver, not in the many available vhost-user backend implementations).
> > > >
> > >
> > > So you mean adding one way to notify guest to resubmit inflight I/O. I
> > > think it's a good idea. But would it be more flexible to implement
> > > this in backend. We can support old guest. And it will be easy to fix
> > > bug or add feature.
> >
> > There are trade-offs with either approach.  In the long run I think it's
> > beneficial minimize non-trivial logic in vhost-user backends.  There
> > will be more vhost-user backend implementations and therefore more bugs
> > if we put the logic into the backend.  This is why I think a simple
> > mechanism for marking the device as needing a reset will be more
> > reliable and less trouble.
> >
> 
> I agree. So is it possible to implement both? In the long run,
> updating guest driver to support this is better. And at that time, the
> logic in backend can be only used to support legacy guest driver.

Yes, in theory both approaches could be available.

Stefan

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