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

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Date:   Wed, 1 Jun 2022 18:58:50 +0000
From:   Parav Pandit <parav@...dia.com>
To:     Jason Wang <jasowang@...hat.com>
CC:     "Michael S. Tsirkin" <mst@...hat.com>,
        Eugenio Pérez <eperezma@...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


> From: Jason Wang <jasowang@...hat.com>
> Sent: Tuesday, May 31, 2022 10:42 PM
> 
> Well, the ability to query the virtqueue state was proposed as another
> feature (Eugenio, please correct me). This should be sufficient for making
> virtio-net to be live migrated.
> 
The device is stopped, it won't answer to this special vq config done here.
Programming all of these using cfg registers doesn't scale for on-chip memory and for the speed.

Next would be to program hundreds of statistics of the 64 VQs through giant PCI config space register in some busy polling scheme.

I can clearly see how all these are inefficient for faster LM.
We need an efficient AQ to proceed with at minimum.

> https://lists.oasis-open.org/archives/virtio-comment/202103/msg00008.html
> 
> > Once the device is stopped, its state cannot be queried further as device
> won't respond.
> > It has limited use case.
> > What we need is to stop non admin queue related portion of the device.