Re: [PATCH] PCI: Disable IOV before pcibios_sriov_disable()

[Gavin Shan <gwshan@linux.vnet.ibm.com>]

On Tue, Mar 21, 2017 at 09:50:06AM +1100, Gavin Shan wrote:
>On Mon, Mar 20, 2017 at 10:46:44AM -0500, Bjorn Helgaas wrote:
>>On Sat, Mar 18, 2017 at 11:19:07AM +1100, Gavin Shan wrote:
>>> On Thu, Mar 09, 2017 at 11:34:32AM +1100, Gavin Shan wrote:
>>> >On Tue, Mar 07, 2017 at 02:15:29PM -0600, Bjorn Helgaas wrote:
>>> >>On Mon, Feb 27, 2017 at 11:18:32AM +1100, Gavin Shan wrote:
>>> >>> The PowerNV platform is the only user of pcibios_sriov_disable().
>>> >>> The IOV BAR could be shifted by pci_iov_update_resource(). The
>>> >>> warning message in the function is printed if the IOV capability
>>> >>> is in enabled (PCI_SRIOV_CTRL_VFE && PCI_SRIOV_CTRL_MSE) state.
>>> >>> 
>>> >>>    pci_disable_sriov
>>> >>>    sriov_disable
>>> >>>    pnv_pci_sriov_disable
>>> >>>    pnv_pci_vf_resource_shift
>>> >>>    pci_update_resource
>>> >>>    pci_iov_update_resource
>>> >>> 
>>> >>> This fixes the issue by disabling IOV capability before calling
>>> >>> pcibios_sriov_disable(). With it, the disabling path matches with
>>> >>> the enabling path: pcibios_sriov_enable() is called before the
>>> >>> IOV capability is enabled.
>>> >>
>>> >>I'm vaguely uncomfortable about this path:
>>> >>
>>> >>  pci_disable_sriov
>>> >>    sriov_disable
>>> >>      pcibios_sriov_disable           # powerpc version
>>> >>	pnv_pci_sriov_disable
>>> >>	  pnv_pci_vf_resource_shift
>>> >>	    res = &dev->resource[i + PCI_IOV_RESOURCES]
>>> >>	    res->start += size * offset
>>> >>	    pci_update_resource
>>> >>	      pci_iov_update_resource
>>> >>	  pnv_pci_vf_release_m64
>>> >>
>>> >>1) "res" is already in the resource tree, so we shouldn't be changing
>>> >>   its start address, because that may make the tree inconsistent,
>>> >>   e.g., the resource may no longer be completely contained in its
>>> >>   parent, it may conflict with a sibling, etc.
>>> >>
>>> >>2) If we update "res->start", shouldn't we update "res->end"
>>> >>   correspondingly?
>>> >>
>>> >>It seems like it'd be better if we didn't update the device resources
>>> >>in the enable/disable paths.  If we could do the resource adjustments
>>> >>earlier, somewhere before we give the device to a driver, it seems
>>> >>like it would avoid these issues.
>>> >>
>>> >>We might have talked about these questions in the past, so I apologize
>>> >>if you've already explained this.  If that's the case, maybe we just
>>> >>need some comments in the code to help the next confused reader.
>>> >>
>>> >
>>> >Bjorn, thanks for review. I agree it's not perfect. We discussed this long
>>> >time ago as I can remember. Let me try to make it a bit more clear: In our
>>> >PHB hardware, there are 16 MMIO BARs. Each of them can be shared by 256 PEs
>>> >(A) and owned exclusively by one PE (B). When VF BAR size is small enough,
>>> >we take (A). Otherwise, we have to take (B). Only when taking (A), we need
>>> >expand/move/shrink the IOV BAR. So lets stick to (A) for discussion here.
>>> >
>>> >Under (A), PF's IOV BAR size is extended to ((256 * (VF BAR size)) when the
>>> >PF is probed. Then the @res, which corresponds to the IOV BAR, is assigned
>>> >and put into the resource tree during resource sizing and assignment stage.
>>> >The IOV capability is going to be enabled by PF's driver or sysfs entry, it
>>> >calls into pnv_pci_sriov_enable() where number of contigous PEs (equal to
>>> >number of VFs to be enabled) are allocated. We shift the IOV BAR base according
>>> >to the starting PE number of the allocated block. Afterewards, the IOV BAR
>>> >is restored when the IOV capability is disabled. So it's all about the PE.
>>> >The IOV BAR's end address isn't touched, we needn't update @res->end when
>>> >restoring the IOV BAR.
>>> >
>>> >In order to avoid moving IOV BAR base address, I need know the the PEs
>>> >for the VFs before resourcd sizing and assignment stage. It means I need
>>> >to reserve PEs in advance, which isn't nice because we never enable the
>>> >VFs. In that case, the PEs are wasted.
>>> >
>>> >Yeah, it's nice to have add some comments in pnv_pci_vf_resource_shift()
>>> >where pci_update_resource() is called. I will post another patch to
>>> >linux-ppcdev and you'll be copied. If you agree, I think you can merge
>>> >this patch as none of the concerns are too much related.
>>> >
>>> 
>>> Sorry, Bjorn, ping! Please let me know if there are more concerns you have.
>>
>>I think we had a misunderstanding -- you mentioned adding some
>>comments and wrote "I will post another patch", and I *thought* you
>>meant you were going to post another version of *this* patch with some
>>updated comments.  So I've been waiting for that updated patch.  But I
>>think you've been waiting for me to merge *this* patch as-is.
>>
>>To avoid having this discussion a third time in the future, I think
>>you should add some comments at the point where you update the
>>resource.  Updating a resource after it's in the resource tree is
>>clearly dangerous, so we need some explanation of why it's sort of OK
>>in this particular case.
>>
>>If you can write a comment and dig up a URL to our previous
>>discussion, I'd like to incorporate that into *this* patch before I
>>merge it.  The sooner we can document this, the less work it will be
>>in the future.
>>
>
>Ok. Sorry for the confusion and that I should looked into the code for more.
>We already had one comment like below in arch/powerpc/platforms/powernv/pci-ioda.c::
>pnv_pci_vf_resource_shift(). I think it's exactly what you like to have, please
>help to confirm. I believe it was added based on your comments long time ago when
>you review the SRIOV (for PowerNV) patches.
>
>        /*
>         * After doing so, there would be a "hole" in the /proc/iomem when
>         * offset is a positive value. It looks like the device return some
>         * mmio back to the system, which actually no one could use it.
>         */
>
>http://www.spinics.net/lists/linux-pci/msg39424.html
>

Bjorn, please let me know if you have concerns.

Thanks,
Gavin