linux-kernel - Re: [PATCH] PCI: pci-stub: accept exceptions to the ID- and class-based matching

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Message-ID: <d777a8f1-4936-eb65-e392-c815bdf6d7e9@redhat.com>
Date:   Tue, 25 Oct 2016 21:40:54 +0200
From:   Laszlo Ersek <lersek@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andrei Grigore <andrei.grg@...il.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jayme Howard <g.prime@...il.com>
Subject: Re: [PATCH] PCI: pci-stub: accept exceptions to the ID- and
 class-based matching

On 10/25/16 21:24, Laszlo Ersek wrote:
> On 10/25/16 20:42, Alex Williamson wrote:

>> FWIW, I think the reason
>> this hasn't been done to date is that PCI bus addresses (except for
>> root bus devices) are not stable.  Depending on the system, the address
>> of a given device may change, not only based on the slot where the
>> device is installed, but whether other devices in other slots are
>> populated.
> 
> I agree.
> 
> However, while the addresses are not stable in the face of hardware
> changes, I think the addresses don't change haphazardly (that is,
> without hardware changes).
> 
> So, if you plug in another card, your current pci-stub.except=...
> parameter might become invalid; but that's not very different from the
> case when you plug in the second instance of a preexistent card right
> now -- then the pci-stub.ids=... filter won't match uniquely anymore,
> and assignment vs. host-side use might not work as intented.

Sorry about the self-followup; I just wanted to add -- although it might
not carry a lot of weight for the host kernel -- that the libvirt domain
XML hard-codes the host-side PCI BDF anyway, for assigned devices:

http://libvirt.org/formatdomain.html#elementsHostDev

"""
  <devices>
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x06' slot='0x02' function='0x0'/>
      </source>
      ...
    </hostdev>
  </devices>

  ...

  source
    The source element describes the device as seen from the host using
    the following mechanism to describe:

    ...

    pci
      PCI devices can only be described by their address.
"""

Thanks
Laszlo