linux-kernel - Re: [PATCH v6 09/10] hisi_acc_vfio_pci: Add support for VFIO live migration

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Message-ID: <20220228234709.GV219866@nvidia.com>
Date:   Mon, 28 Feb 2022 19:47:09 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>,
        "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>,
        "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 v6 09/10] hisi_acc_vfio_pci: Add support for VFIO live
 migration

On Mon, Feb 28, 2022 at 02:20:34PM -0700, Alex Williamson wrote:

> > Unless you think we should block it.
> 
> What's the meaning of initial_bytes and dirty_bytes while in
> STOP_COPY?

Same as during pre-copy - both numbers are the bytes remaining to be
read() from the FD in each bucket. They should continue to decline as
read() progresses regardless of what state the data_fd is in.

The only special thing about STOP_COPY is that dirty_bytes should not
increase as the device should not be generating new dirty data.

How about:

 * Drivers should attempt to return estimates so that initial_bytes +
 * dirty_bytes matches the amount of data an immediate transition to STOP_COPY
 * will require to be streamed. While in STOP_COPY the initial_bytes
 * and dirty_bytes should continue to be decrease as the data_fd
 * progresses streaming out the data.

Remove the 'in the precopy phase' from the first sentance

Adjust the last paragraph as:

+ * returning readable. ENOMSG may not be returned in STOP_COPY. Support
+ * for this ioctl is required when VFIO_MIGRATION_PRE_COPY is set.

Jason