Message-ID: <4561FB62-054B-4E2D-9FDD-780FDB1374F6@microsoft.com>
Date:   Fri, 23 Mar 2018 16:09:14 +0000
From:   Sridhar Pitchai <Sridhar.Pitchai@...rosoft.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jake Oshins <jakeo@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3]PCI: hv: fix PCI-BUS domainID corruption

    > Previously, when using the bond driver, unique and persistent VF NIC name
    > was required, so we used serial number as PCI domain which is included as
    > part of the VF NIC name.  Transparent SRIOV mode puts VF NIC based on MAC
    > match as a slave of synthetic NIC, so VF NIC’s name is no longer important.
    
    Hi Sridhar,
    
    A few hints about submitting patches more efficiently:
    
    1) You never have to ask "Are we OK with the explanation?  If so, I'll
    send a patch with updated changelog."  That forces an extra
    round-trip.  Simply post a new version with your proposed update.  If
    Lorenzo has more questions, he'll say so and you can do another
    version.
    
    2) When Lorenzo is asking for clarification, he's not really asking
    for the clarification in an email response, because the email thread
    will soon be forgotten and lost in the archives.  What we really want
    is for the permanent git changelog to make sense to someone in the
    future.  The easiest way is to post a new patch version with a
    revised changelog that answers the questions.
    
    3) Please capitalize and punctuate consistently with previous history,
    e.g., "PCI: hv: Fix domain ID corruption" for your title, "SR-IOV"
    (not "SRIOV") and "bus" (not "BUS") in changelog.  Both "para-virtual"
    and "paravirtual" are used in the kernel, but "paravirtual" is much
    more common.  Run "git log" and "git log --oneline" on your file and
    follow the same style.
    
    4) When you reference a previous commit, please use this style:
    0c195567a8f6 ("netvsc: transparent VF management"), i.e., 12-char SHA1
    followed by title.  You seem to have removed some spaces from the
    commit you mention in the "Fixes" tag.
    
    And a few content questions/observations of my own:
    
    1) "Fix domain ID corruption" isn't a very good title because it
    suggests you're fixing a memory corruption or similar defect.  But in
    fact, I think you're removing something that used to be a feature
    (added by 4a9b0933bdfc ("PCI: hv: Use device serial number as PCI
    domain")) but is now no longer needed and in fact now causes a
    problem.
    
    2) Your changelog does mention 4a9b0933bdfc, which is good, but
    there must be some other <commit X> that makes it safe to remove
    4a9b0933bdfc, i.e., <commit X> removes the need for using the device
    serial number as the PCI domain.  <Commit X> *must* be mentioned in
    the changelog.  Otherwise, people may backport this patch to a kernel
    that doesn't include <commit X>, and things will break.
    
    3) I don't understand what you mean by "transparent SR-IOV mode".  Is
    that something different than regular SR-IOV?  If so, what exactly is
    the difference?  I don't think the PCIe specs mention a "transparent
    mode", so is it a Hyper-V thing?  It seems important, but I don't see
    any pci-hyperv.c commits that mention it.
    
    Here's a stab at the sort of changelog I would be looking for.
    Obviously I don't understand much about Hyper-V and pci-hyperv.c, so
    please correct the things I got wrong:
    
      When Linux runs as a guest in a Hyper-V VM, pci-hyperv.c
      paravirtualizes access to PCI devices assigned to the guest.  For
      each of those devices, hv_pci_probe() creates a virtual PCI bus in
      its own unique PCI domain.
    
      4a9b0933bdfc ("PCI: hv: Use device serial number as PCI domain")
      overrode that unique PCI domain to be the Hyper-V device serial
      number to make device names more convenient <or whatever the real
      reason is; I don't quite understand this part>.
    
      One problem with 4a9b0933bdfc is that the Hyper-V device serial
      number is not necessarily unique, so we may end up with two buses
      with the same domain and bus number, and adding the second bus
      fails.
    
      We no longer need to override the PCI domain numbers because <commit
      X> removed the need for that.
    
      Revert 4a9b0933bdfc ("PCI: hv: Use device serial number as PCI
      domain") so we can reliably support multiple devices being assigned
      to a guest.
    
      This revert should only be backported to kernels that contain
      <commit X>.
    
    Bjorn

Thanks for your comments Bjorn. I took some time to go over the all the
comments and sending a new version of the patch with all the comments 
incorporated.

Thanks
Sridhar    

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