netdev - RE: [PATCH V2 mlx5-next 12/14] vfio/mlx5: Implement vfio

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Date:   Tue, 2 Nov 2021 11:19:18 +0000
From:   Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>
CC:     Cornelia Huck <cohuck@...hat.com>,
        Yishai Hadas <yishaih@...dia.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "saeedm@...dia.com" <saeedm@...dia.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "leonro@...dia.com" <leonro@...dia.com>,
        "kwankhede@...dia.com" <kwankhede@...dia.com>,
        "mgurtovoy@...dia.com" <mgurtovoy@...dia.com>,
        "maorg@...dia.com" <maorg@...dia.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        liulongfang <liulongfang@...wei.com>,
        "Zengtao (B)" <prime.zeng@...ilicon.com>
Subject: RE: [PATCH V2 mlx5-next 12/14] vfio/mlx5: Implement vfio_pci driver
 for mlx5 devices



> -----Original Message-----
> From: Jason Gunthorpe [mailto:jgg@...dia.com]
> Sent: 01 November 2021 17:25
> To: Alex Williamson <alex.williamson@...hat.com>; Shameerali Kolothum
> Thodi <shameerali.kolothum.thodi@...wei.com>
> Cc: Cornelia Huck <cohuck@...hat.com>; Yishai Hadas <yishaih@...dia.com>;
> bhelgaas@...gle.com; saeedm@...dia.com; linux-pci@...r.kernel.org;
> kvm@...r.kernel.org; netdev@...r.kernel.org; kuba@...nel.org;
> leonro@...dia.com; kwankhede@...dia.com; mgurtovoy@...dia.com;
> maorg@...dia.com; Dr. David Alan Gilbert <dgilbert@...hat.com>
> Subject: Re: [PATCH V2 mlx5-next 12/14] vfio/mlx5: Implement vfio_pci driver
> for mlx5 devices
> 
> On Fri, Oct 29, 2021 at 04:06:21PM -0600, Alex Williamson wrote:
> 
> > > Right now we are focused on the non-P2P cases, which I think is a
> > > reasonable starting limitation.
> >
> > It's a reasonable starting point iff we know that we need to support
> > devices that cannot themselves support a quiescent state.  Otherwise it
> > would make sense to go back to work on the uAPI because I suspect the
> > implications to userspace are not going to be as simple as "oops, can't
> > migrate, there are two devices."  As you say, there's a universe of
> > devices that run together that don't care about p2p and QEMU will be
> > pressured to support migration of those configurations.
> 
> I agree with this, but I also think what I saw in the proposed hns
> driver suggests it's HW cannot do quiescent, if so this is the first
> counter-example to the notion it is a universal ability?
> 
> hns people: Can you put your device in a state where it is operating,
> able to accept and respond to MMIO, and yet guarentees it generates no
> DMA transactions?

AFAIK, I am afraid we cannot guarantee that as per our current implementation.
At present in !RUNNING state we are putting the device in to a PAUSE state so it
will complete the current request and keep the remaining ones in queue. But it can
still receive a new request which will trigger the PAUSE state exit and resume the
operation.

So I guess, it is possible to corrupt the migration if user space misbehaves.

Thanks,
Shameer