linux-kernel - Re: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

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Message-ID: <CAPcyv4iYHA1acfo=+fTk+U_TrLbSWJjA6v4oeTXgVYDTrnCoGw@mail.gmail.com>
Date:   Fri, 6 Nov 2020 15:47:00 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     "Raj, Ashok" <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "Dey, Megha" <megha.dey@...el.com>,
        "maz@...nel.org" <maz@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...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>,
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Subject: Re: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

On Fri, Nov 6, 2020 at 9:51 AM Jason Gunthorpe <jgg@...dia.com> wrote:
[..]
> > This is true for IMS as well. But probably not implemented in the kernel as
> > such. From a HW point of view (take idxd for instance) the facility is
> > available to native OS as well. The early RFC supported this for native.
>
> I can't follow what you are trying to say here.

I'm having a hard time following the technical cruxes of this debate.
I grokked your feedback on the original IMS proposal way back at the
beginning of this effort (pre-COVID even!), so maybe I can mediate
here as well. Although, SIOV is that much harder for me to spell than
IMS, so bear with me.

> Dave said the IMS cap was to indicate that the VMM supported emulation
> of IMS so that the VMM can do the MSI addr/data translation as part of
> the emulation.
>
> I'm saying emulation will be too horrible for our devices that don't
> require *any* emulation.

This part I think I understand, i.e. why spend any logic emulating IMS
as MSI since the IMS capability can be a paravirtualized interface
from guest to VMM with none of the compromises that MSI would enforce.
Did I get that right?

> It is a bad architecture. The platform needs to handle this globally
> for all devices, not special hacky emulations things custom made for
> every device out there.

I confess I don't quite understand the shape of what "platform needs
to handle this globally" means, but I understand the desired end
result of "no emulation added where not needed". However, would this
mean that the bare-metal idxd driver can not be used directly in the
guest without modification? For example, as I understand from talking
to Ashok, idxd has some device events like error notification hard
wired to MSI while data patch interrupts are IMS. So even if the IMS
side does not hook up MSI emulation doesn't idxd still need MSI
emulation to reuse the bare metal driver directly?

> > Native devices can have both MSIx and IMS capability. But as I
> > understand this isn't how we have partitioned things in SW today. We
> > left IMS only for mdev's. And I agree this would be very useful.
>
> That split is just some decision idxd did, we are thinking about doing
> other things in our devices.

Where does the collision happen between what you need for a clean
implementation of an IMS-like capability (/me misses his "dev-msi"
name that got thrown out in the Thomas rewrite), and emulation needed
to not have VF special casing in the idxd driver.

Also feel free to straighten me out (Jason or Ashok) if I've botched
the understanding of this.