linux-kernel - Re: [PATCH v7 09/10] hisi_acc_vfio_pci: Add support for VFIO live migration

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Date:   Thu, 3 Mar 2022 09:19:38 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "mgurtovoy@...dia.com" <mgurtovoy@...dia.com>,
        "yishaih@...dia.com" <yishaih@...dia.com>,
        Linuxarm <linuxarm@...wei.com>,
        liulongfang <liulongfang@...wei.com>,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        "Wangzhou (B)" <wangzhou1@...ilicon.com>
Subject: Re: [PATCH v7 09/10] hisi_acc_vfio_pci: Add support for VFIO live
 migration

On Thu, Mar 03, 2022 at 01:17:05PM +0000, Shameerali Kolothum Thodi wrote:
> > Tthere is a scenario that transfers only QM_MATCH_SIZE in stop_copy?
> > This doesn't seem like a good idea, I think you should transfer a
> > positive indication 'this device is not ready' instead of truncating
> > the stream. A truncated stream should not be a valid stream.
> > 
> > ie always transfer the whole struct.
> 
> We could add a 'qm_state' and return the whole struct. But the rest
> of the struct is basically invalid if qm_state = QM_NOT_REDAY.

This seems like the right thing to do to me

> > > Looks like setting the total_length = 0 in STOP_COPY is a better
> > > solution(If there are no other issues with that) as it will avoid
> > > grabbing the state_mutex as you mentioned above.
> > 
> > That seems really weird, I wouldn't recommend doing that..
> 
> Does that mean we don't support a zero data transfer in STOP_COPY?

total_length should not go backwards

> The concern is if we always transfer the whole struct, we end up reading
> and writing the whole thing even if most of the data is invalid.

Well, you can't know if the device is not ready or not until the
reaching STOP_COPY, so you have to transfer something to avoid
truncating the data stream. The state here is tiny, is the extra
transfer a real worry?

Jason