From: Alex Williamson <alex.williamson@redhat.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-crypto@vger.kernel.org, linux-pci@vger.kernel.org,
cohuck@redhat.com, mgurtovoy@nvidia.com, yishaih@nvidia.com,
linuxarm@huawei.com, liulongfang@huawei.com,
prime.zeng@hisilicon.com, jonathan.cameron@huawei.com,
wangzhou1@hisilicon.com
Subject: Re: [PATCH v7 07/10] vfio: Extend the device migration protocol with PRE_COPY
Date: Wed, 2 Mar 2022 20:47:52 -0700 [thread overview]
Message-ID: <20220302204752.71ea8b32.alex.williamson@redhat.com> (raw)
In-Reply-To: <20220303000528.GW219866@nvidia.com>
On Wed, 2 Mar 2022 20:05:28 -0400
Jason Gunthorpe <jgg@nvidia.com> wrote:
> On Wed, Mar 02, 2022 at 01:31:59PM -0700, Alex Williamson wrote:
> > > + * initial_bytes reflects the estimated remaining size of any initial mandatory
> > > + * precopy data transfer. When initial_bytes returns as zero then the initial
> > > + * phase of the precopy data is completed. Generally initial_bytes should start
> > > + * out as approximately the entire device state.
> >
> > What is "mandatory" intended to mean here? The user isn't required to
> > collect any data from the device in the PRE_COPY states.
>
> If the data is split into initial,dirty,trailer then mandatory means
> that first chunk.
But there's no requirement to read anything in PRE_COPY, so initial
becomes indistinguishable from trailer and dirty doesn't exist.
> > "The vfio_precopy_info data structure returned by this ioctl provides
> > estimates of data available from the device during the PRE_COPY states.
> > This estimate is split into two categories, initial_bytes and
> > dirty_bytes.
> >
> > The initial_bytes field indicates the amount of static data available
> > from the device. This field should have a non-zero initial value and
> > decrease as migration data is read from the device.
>
> static isn't great either, how about just say 'minimum data available'
'initial precopy data-set'?
> > Userspace may use the combination of these fields to estimate the
> > potential data size available during the PRE_COPY phases, as well as
> > trends relative to the rate the device is dirtying it's internal
> > state, but these fields are not required to have any bearing relative
> > to the data size available during the STOP_COPY phase."
>
> That last is too strong. I would just drop starting at but.
>
> The message to communicate is the device should allow dirty_bytes to
> reach 0 during the PRE_COPY phases if everything is is idle. Which
> tells alot about how to calculate it.
>
> It is all better otherwise
>
> > > + * 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.
> >
> > I think previous discussions have proven this false, we expect trailing
> > data that is only available in STOP_COPY, we cannot bound the size of
> > that data, and dirty_bytes is not intended to expose data that cannot
> > be retrieved during the PRE_COPY phases. Thanks,
>
> It was written assuming the stop_copy trailer is small.
We have no basis to make that assertion. We've agreed that precopy can
be used for nothing more than a compatibility test, so we could have a
vGPU with a massive framebuffer and no ability to provide dirty
tracking implement precopy only to include the entire framebuffer in
the trailing STOP_COPY data set. Per my understanding and the fact
that we cannot enforce any heuristics regarding the size of the tailer
relative to the pre-copy data set, I think the above strongly phrased
sentence is necessary to understand the limitations of what this ioctl
is meant to convey. Thanks,
Alex
next prev parent reply other threads:[~2022-03-03 3:48 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-02 17:28 [PATCH v7 00/10] vfio/hisilicon: add ACC live migration driver Shameer Kolothum
2022-03-02 17:28 ` [PATCH v7 01/10] crypto: hisilicon/qm: Move the QM header to include/linux Shameer Kolothum
2022-03-02 18:48 ` John Garry
2022-03-02 20:39 ` Shameerali Kolothum Thodi
2022-03-02 17:28 ` [PATCH v7 02/10] crypto: hisilicon/qm: Move few definitions to common header Shameer Kolothum
2022-03-02 18:55 ` John Garry
2022-03-02 20:41 ` Shameerali Kolothum Thodi
2022-03-02 17:28 ` [PATCH v7 03/10] hisi_acc_qm: Move VF PCI device IDs " Shameer Kolothum
2022-03-02 17:28 ` [PATCH v7 04/10] hisi_acc_vfio_pci: add new vfio_pci driver for HiSilicon ACC devices Shameer Kolothum
2022-03-02 19:02 ` John Garry
2022-03-02 20:32 ` Shameerali Kolothum Thodi
2022-03-02 17:28 ` [PATCH v7 05/10] hisi_acc_vfio_pci: Restrict access to VF dev BAR2 migration region Shameer Kolothum
2022-03-02 17:28 ` [PATCH v7 06/10] hisi_acc_vfio_pci: Add helper to retrieve the struct pci_driver Shameer Kolothum
2022-03-02 17:29 ` [PATCH v7 07/10] vfio: Extend the device migration protocol with PRE_COPY Shameer Kolothum
2022-03-02 20:31 ` Alex Williamson
2022-03-03 0:05 ` Jason Gunthorpe
2022-03-03 3:47 ` Alex Williamson [this message]
2022-03-03 13:01 ` Jason Gunthorpe
2022-03-03 15:20 ` Alex Williamson
2022-03-03 18:05 ` Shameerali Kolothum Thodi
2022-03-03 19:59 ` Alex Williamson
2022-03-03 23:49 ` Jason Gunthorpe
2022-03-04 19:56 ` Alex Williamson
2022-03-02 17:29 ` [PATCH v7 08/10] crypto: hisilicon/qm: Set the VF QM state register Shameer Kolothum
2022-03-02 17:29 ` [PATCH v7 09/10] hisi_acc_vfio_pci: Add support for VFIO live migration Shameer Kolothum
2022-03-03 0:21 ` Jason Gunthorpe
2022-03-03 12:57 ` Shameerali Kolothum Thodi
2022-03-03 13:04 ` Jason Gunthorpe
2022-03-03 13:17 ` Shameerali Kolothum Thodi
2022-03-03 13:19 ` Jason Gunthorpe
2022-03-03 13:27 ` Shameerali Kolothum Thodi
2022-03-02 17:29 ` [PATCH v7 10/10] hisi_acc_vfio_pci: Use its own PCI reset_done error handler Shameer Kolothum
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