linux-kernel - Re: [PATCH RFC v2 00/18] Add VFIO mediated device support and DEV-MSI support for the idxd driver

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Message-ID: <20200818115003.GM1152540@nvidia.com>
Date:   Tue, 18 Aug 2020 08:50:03 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
CC:     Alex Williamson <alex.williamson@...hat.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "Dey, Megha" <megha.dey@...el.com>,
        "maz@...nel.org" <maz@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Lu, Baolu" <baolu.lu@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Lin, Jing" <jing.lin@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
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        "eric.auger@...hat.com" <eric.auger@...hat.com>,
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        "Hansen, Dave" <dave.hansen@...el.com>,
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        "Ortiz, Samuel" <samuel.ortiz@...el.com>,
        "Hossain, Mona" <mona.hossain@...el.com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH RFC v2 00/18] Add VFIO mediated device support and
 DEV-MSI support for the idxd driver

On Tue, Aug 18, 2020 at 01:09:01AM +0000, Tian, Kevin wrote:
> The difference in my reply is not just about the implementation gap
> of growing a userspace DMA framework to a passthrough framework.
> My real point is about the different goals that each wants to achieve.
> Userspace DMA is purely about allowing userspace to directly access
> the portal and do DMA, but the wq configuration is always under kernel 
> driver's control. On the other hand, passthrough means delegating full 
> control of the wq to the guest and then associated mock-up (live migration,
> vSVA, posted interrupt, etc.) for that to work. I really didn't see the
> value of mixing them together when there is already a good candidate
> to handle passthrough...

In Linux a 'VM' and virtualization has always been a normal system
process that uses a few extra kernel features. This has been more or
less the cornerstone of that design since the start.

In that view it doesn't make any sense to say that uAPI from idxd that
is useful for virtualization somehow doesn't belong as part of the
standard uAPI.

Especially when it is such a small detail like what APIs are used to
configure the wq.

For instance, what about suspend/resume of containers using idxd?
Wouldn't you want to have the same basic approach of controlling the
wq from userspace that virtualization uses?

Jason