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

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Date:   Sun, 8 Nov 2020 19:29:50 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     David Woodhouse <dwmw2@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "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>,
        "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 Sun, Nov 08, 2020 at 11:47:13PM +0100, Thomas Gleixner wrote:

> OTOH, what's the chance that a guest runs on something which
> 
>   1) Does not have X86_FEATURE_HYPERVISOR set in cpuid 1/EDX
> 
> and
> 
>   2) Cannot be identified as Xen domain
> 
> and
> 
>   3) Does not have a DMI vendor entry which identifies the
>      virtualization solution (we don't use that today, but
>      adding that table is trivial enough)
> 
> and
> 
>   4) Has such an IMS device passed through?
> 
> Possible, yes. Likely, no. Do we care?

This is exactly my thinking too. IMS is still very new, if we add some
platform flag to disable it then yes there are broken cases but enough
options for an unlucky user to deal with it:

 - Have their VMM set X86_FEATURE_HYPERVISOR
 - Updating the VMM to set the global disable flag
 - Add some "disable_subdevice_msi" kernel comand line flag in the guest

In exchange we get a much cleaner architecture for the next 10 years..

Jason