linux-kernel - Re: [PATCH 0/2] PCI: Workaround for bus reset on Cavium cn8xxx root ports

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Message-ID: <20170523161524.565357ea@w520.home>
Date:   Tue, 23 May 2017 16:15:24 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     David Daney <ddaney@...iumnetworks.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        David Daney <david.daney@...ium.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        Jon Masters <jcm@...hat.com>,
        Robert Richter <robert.richter@...ium.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/2] PCI: Workaround for bus reset on Cavium cn8xxx root
 ports

On Tue, 23 May 2017 14:22:01 -0700
David Daney <ddaney@...iumnetworks.com> wrote:

> On 05/23/2017 02:04 PM, Alex Williamson wrote:
> > On Tue, 23 May 2017 15:47:50 -0500
> > Bjorn Helgaas <helgaas@...nel.org> wrote:
> >   
> >> On Mon, May 15, 2017 at 05:17:34PM -0700, David Daney wrote:  
> >>> With the recent improvements in arm64 and vfio-pci, we are seeing
> >>> failures like this (on cn8890 based systems):
> >>>
> >>> [  235.622361] Unhandled fault: synchronous external abort (0x96000210) at 0xfffffc00c1000100
> >>> [  235.630625] Internal error: : 96000210 [#1] PREEMPT SMP
> >>> .
> >>> .
> >>> .
> >>> [  236.208820] [<fffffc0008411250>] pci_generic_config_read+0x38/0x9c
> >>> [  236.214992] [<fffffc0008435ed4>] thunder_pem_config_read+0x54/0x1e8
> >>> [  236.221250] [<fffffc0008411620>] pci_bus_read_config_dword+0x74/0xa0
> >>> [  236.227596] [<fffffc000841853c>] pci_find_next_ext_capability.part.15+0x40/0xb8
> >>> [  236.234896] [<fffffc0008419428>] pci_find_ext_capability+0x20/0x30
> >>> [  236.241068] [<fffffc0008423e2c>] pci_restore_vc_state+0x34/0x88
> >>> [  236.246979] [<fffffc000841af3c>] pci_restore_state.part.37+0x2c/0x1fc
> >>> [  236.253410] [<fffffc000841b174>] pci_dev_restore+0x4c/0x50
> >>> [  236.258887] [<fffffc000841b19c>] pci_bus_restore+0x24/0x4c
> >>> [  236.264362] [<fffffc000841c2dc>] pci_try_reset_bus+0x7c/0xa0
> >>> [  236.270021] [<fffffc00060a1ab0>] vfio_pci_ioctl+0xc34/0xc3c [vfio_pci]
> >>> [  236.276547] [<fffffc0005eb0410>] vfio_device_fops_unl_ioctl+0x20/0x30 [vfio]
> >>> [  236.283587] [<fffffc000824b314>] do_vfs_ioctl+0xac/0x744
> >>> [  236.288890] [<fffffc000824ba30>] SyS_ioctl+0x84/0x98
> >>> [  236.293846] [<fffffc0008082ca0>] __sys_trace_return+0x0/0x4
> >>>
> >>> These are caused by the inability of the PCIe root port and Intel
> >>> e1000e to sucessfully do a bus reset.
> >>>
> >>> The proposed fix is to not do a bus reset on these systems.
> >>>
> >>> David Daney (2):
> >>>    PCI: Allow PCI_DEV_FLAGS_NO_BUS_RESET to be used on bus device.
> >>>    PCI: Avoid bus reset for Cavium cn8xxx root ports.
> >>>
> >>>   drivers/pci/pci.c    | 4 ++++
> >>>   drivers/pci/quirks.c | 8 ++++++++
> >>>   2 files changed, 12 insertions(+)  
> >>
> >> Applied with Eric's reviewed-by and typo fixes to pci/virtualization for
> >> v4.13, thanks!  
> > 
> > Hmm, well let me again express my concerns that I'm really not sure how
> > to support this since it removes our last opportunity to reset devices
> > that may otherwise have no reset mechanism.  Certain classes of devices
> > are entirely unsupportable for the code path indicated above without a
> > bus reset.  If we have an endpoint device that goes bonkers at a bus
> > reset, at least we know it's going to behave just as poorly no matter
> > what the host platform.  This series allows endpoints that work
> > perfectly well on one host to be handled differently on another.  
> 
> Yes that is correct.  We choose not to crash the system.  I'm not sure 
> what you are suggesting as an alternative.

It's a valid solution, but my concern is that it's not without
consequences and I didn't get the impression that was fully recognized,
so I wonder if there are better options.

> If a PCI device doesn't work with vfio-pci in such a system, my 
> suggestion would be not to use vfio-pci with the device in that system.

And I can use this as the official support statement from Cavium when
that occurs?
 
> >  It
> > certainly suggests something non-spec compliant about the root port
> > implementation and I wish there was more analysis about exactly what
> > that problem is since this is coming from the hardware vendor.  
> 
> There are two main possibilities here:
> 
> 1) Some (but not all) Intel e1000e and LSI HBA devices are non-spec 
> compliant.
> 
> 2) Cavium root port is non-spec compliant.
> 
> If #1 turns out to be true, would you suggest blacklisting e1000e on all 
> systems, including Intel based servers?

Since we haven't heard anything to the contrary, I think we can also
assume that AMD root ports are compatible.
 
> If #2 turns out to be true would you still object to the patch?

My hope would be that such analysis would help us understand what's
really happening and perhaps present a less drastic workaround.  Is the
point at which we crash simply the first read from the device?  Is the
link state shown as stable at this point?  Is there any chance that we
have a timing issue and an additional delay could resolve it?  Is there
some manipulation of the link state before or after the bus reset that
produces different results?  If such options have been exhausted, then
I guess I have no objection, or at least no better solution, and I'll
keep an eye out for any fallout.  Thanks,

Alex