linux-kernel - Re: [PATCH v4 0/4] Implement vdpasim stop operation

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Message-ID: <CACGkMEu1YenjBHAssP=FvKX6WxDQ5Aa50r-BsnkfR4zqNTk6hg@mail.gmail.com>
Date:   Fri, 27 May 2022 10:26:35 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Parav Pandit <parav@...dia.com>
Cc:     Eugenio Pérez <eperezma@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "martinh@...inx.com" <martinh@...inx.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        "martinpo@...inx.com" <martinpo@...inx.com>,
        "lvivier@...hat.com" <lvivier@...hat.com>,
        "pabloc@...inx.com" <pabloc@...inx.com>,
        Eli Cohen <elic@...dia.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Xie Yongji <xieyongji@...edance.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Zhang Min <zhang.min9@....com.cn>,
        Wu Zongyong <wuzongyong@...ux.alibaba.com>,
        "lulu@...hat.com" <lulu@...hat.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>,
        "Piotr.Uminski@...el.com" <Piotr.Uminski@...el.com>,
        Si-Wei Liu <si-wei.liu@...cle.com>,
        "ecree.xilinx@...il.com" <ecree.xilinx@...il.com>,
        "gautam.dawar@....com" <gautam.dawar@....com>,
        "habetsm.xilinx@...il.com" <habetsm.xilinx@...il.com>,
        "tanuj.kamde@....com" <tanuj.kamde@....com>,
        "hanand@...inx.com" <hanand@...inx.com>,
        "dinang@...inx.com" <dinang@...inx.com>,
        Longpeng <longpeng2@...wei.com>
Subject: Re: [PATCH v4 0/4] Implement vdpasim stop operation

On Thu, May 26, 2022 at 8:54 PM Parav Pandit <parav@...dia.com> wrote:
>
>
>
> > From: Eugenio Pérez <eperezma@...hat.com>
> > Sent: Thursday, May 26, 2022 8:44 AM
>
> > Implement stop operation for vdpa_sim devices, so vhost-vdpa will offer
> >
> > that backend feature and userspace can effectively stop the device.
> >
> >
> >
> > This is a must before get virtqueue indexes (base) for live migration,
> >
> > since the device could modify them after userland gets them. There are
> >
> > individual ways to perform that action for some devices
> >
> > (VHOST_NET_SET_BACKEND, VHOST_VSOCK_SET_RUNNING, ...) but there
> > was no
> >
> > way to perform it for any vhost device (and, in particular, vhost-vdpa).
> >
> >
> >
> > After the return of ioctl with stop != 0, the device MUST finish any
> >
> > pending operations like in flight requests. It must also preserve all
> >
> > the necessary state (the virtqueue vring base plus the possible device
> >
> > specific states) that is required for restoring in the future. The
> >
> > device must not change its configuration after that point.
> >
> >
> >
> > After the return of ioctl with stop == 0, the device can continue
> >
> > processing buffers as long as typical conditions are met (vq is enabled,
> >
> > DRIVER_OK status bit is enabled, etc).
>
> Just to be clear, we are adding vdpa level new ioctl() that doesn’t map to any mechanism in the virtio spec.

We try to provide forward compatibility to VIRTIO_CONFIG_S_STOP. That
means it is expected to implement at least a subset of
VIRTIO_CONFIG_S_STOP.

>
> Why can't we use this ioctl() to indicate driver to start/stop the device instead of driving it through the driver_ok?

So the idea is to add capability that does not exist in the spec. Then
came the stop/resume which can't be done via DRIVER_OK. I think we
should only allow the stop/resume to succeed after DRIVER_OK is set.

> This is in the context of other discussion we had in the LM series.

Do you see any issue that blocks the live migration?

Thanks