linux-kernel - Re: [PATCH V2 3/7] vhost-vdpa: VHOST_NEW

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Message-ID: <9686e6ff-70fb-46a2-9828-720379b350fb@oracle.com>
Date: Mon, 15 Jul 2024 11:38:27 -0400
From: Steven Sistare <steven.sistare@...cle.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: virtualization@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>, Si-Wei Liu <si-wei.liu@...cle.com>,
        Eugenio Perez Martin <eperezma@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Dragos Tatulea <dtatulea@...dia.com>
Subject: Re: [PATCH V2 3/7] vhost-vdpa: VHOST_NEW_OWNER

On 7/15/2024 10:38 AM, Michael S. Tsirkin wrote:
> On Mon, Jul 15, 2024 at 10:29:26AM -0400, Steven Sistare wrote:
>> On 7/15/2024 5:07 AM, Michael S. Tsirkin wrote:
>>> On Fri, Jul 12, 2024 at 06:18:49AM -0700, Steve Sistare wrote:
>>>> Add an ioctl to transfer file descriptor ownership and pinned memory
>>>> accounting from one process to another.
>>>>
>>>> This is more efficient than VHOST_RESET_OWNER followed by VHOST_SET_OWNER,
>>>> as that would unpin all physical pages, requiring them to be repinned in
>>>> the new process.  That would cost multiple seconds for large memories, and
>>>> be incurred during a virtual machine's pause time during live update.
>>>>
>>>> Signed-off-by: Steve Sistare <steven.sistare@...cle.com>
>>>
>>> Please, we just need to switch to use iommufd for pinning.
>>> Piling up all these hacks gets us nowhere.
>>
>> I am working on iommufd kernel interfaces and QEMU changes.  But who is working
>> on iommufd support for vdpa? If no one, or not for years, then adding these
>> small interfaces to vdpa plugs a signficant gap in live update coverage.
>>
>> FWIW, the iommufd interfaces for live update will look much the same: change owner
>> and pinned memory accounting, and update virtual addresses.  So adding that to vdpa
>> will not make it look like an odd duck.
>>
>> - Steve
> 
> I think that no one is working on it - Cindy posted some rfcs in January
> ("vhost-vdpa: add support for iommufd").  Feel free to pick that up.
> What you described is just more of a reason not to duplicate this code.
> And it's always the same: a small extension here, a small extension there.
> If you can make do with existing kernel interfaces, fine,
> one can argue that userspace code is useful to support existing kernels.

Respectfully, I will not be volunteering to take that on.  My work funnel to
deliver live update is already too wide.  I hope that folks on this distribution
find the functionality interesting and useful enough to continue to review this
current proposal.

- Steve