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

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Message-ID: <20201104124017.GW2620339@nvidia.com>
Date:   Wed, 4 Nov 2020 08:40:17 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
CC:     "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>,
        "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>,
        "jing.lin@...el.com" <jing.lin@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
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Subject: Re: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

On Wed, Nov 04, 2020 at 03:41:33AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Tuesday, November 3, 2020 8:44 PM
> > 
> > On Tue, Nov 03, 2020 at 02:49:27AM +0000, Tian, Kevin wrote:
> > 
> > > > There is a missing hypercall to allow the guest to do this on its own,
> > > > presumably it will someday be fixed so IMS can work in guests.
> > >
> > > Hypercall is VMM specific, while IMS cap provides a VMM-agnostic
> > > interface so any guest driver (if following the spec) can seamlessly
> > > work on all hypervisors.
> > 
> > It is a *VMM* issue, not PCI. Adding a PCI cap to describe a VMM issue
> > is architecturally wrong.
> > 
> > IMS *can not work* in any hypervsior without some special
> > hypercall. Just block it in the platform code and forget about the PCI
> > cap.
> > 
> 
> It's per-device thing instead of platform thing. If the VMM understands
> the IMS format of a specific device and virtualize it to the guest,

Please no! Adding device specific emulation is just going down deeper
into this bad architecture.

Interrupts is a platform issue. Using emulation of MSI to dynamically
insert vectors to a VM was a reasonable, but hacky thing. Now it needs
proper platform support.

Jason