Re: [REGRESSION, bisect] pci: cxgb4 probe fails after commit 104daa71b3961434 ("PCI: Determine actual VPD size on first access")

[Ganesh Goudar <ganeshgr@chelsio.com>]

+Hannes Reinecke

On Tuesday, January 01/23/18, 2018 at 17:59:09 +0530, Arjun Vynipadath wrote:
> Sending on behalf of "Casey Leedom <leedom@chelsio.com>"
> 
> Way back on April 11, 2016 we reported a regression in Linux kernel 4.6-rc2 
> brought on by kernel.org commit 104daa71b396. This commit calculates the 
> size of a PCI Device's VPD area by parsing the VPD Structure at offset 0x000, 
> and restricts accesses to the VPD to that computed size.
> 
> Our devices have a second VPD structure which is located starting at offset 
> 0x400 which is the "real" VPD[1].  The 104daa71b396 commit (plus a follow on 
> commit 408641e93aa5) caused efforts to read past the end of that computed 
> length of the VPD to return silently without error leaving stack junk in the 
> VPD read buffers.
> 
> We introduced kernel.org commit cb92148b to allow a driver to tell the 
> kernel how large the VPD area really is, introducing a new API 
> pci_set_vpd_size() for this purpose.
> 
> Now we've discovered a new subtlety to the problem.
> 
> We have a KVM Hypervisor running a 4.9.70 kernel.  So it has all of the 
> above commits.  When we attach our Physical Function 4 to a Virtual Machine 
> and attempt to run cxgb4 in that VM, we see the problem again.  The issue is 
> that all of the VM Guest OS's efforts to access the PCIe VPD Capability are 
> trapped into the KVM 4.9.70 kernel and executed there, with the results 
> routed back to the VM Guest OS.  The cxgb4 driver in the VM Guest OS uses 
> the new pci_set_vpd_size() to notify the OS of the true size of the VPD, but 
> that information of course is never sent to the KVM 4.9.70 Hypervisor. 
> (And, truth be told, if the Guest OS were older than 4.6, it wouldn't even 
> know that it needed to do this.)  The result is that again we get silent VPD 
> read failures with random stack garbage in the VPD read buffers. (sigh) 
> 
> It strikes me that the only way to handle this issue is to have KVM 
> circumvent the VPD-Size Restricted logic which was added in kernel.org 
> commits 104daa71b396 and 408641e93aa5.  Maybe via a __pci_read_vpd() or 
> similar API.  But we are open to other suggestions.
> 
> Thoughts?
> 
> Casey.
> 
> [1] Chelsio adapters actually have two VPD structures stored in the VPD.  An
>     abbreviated on at Offset 0x0 and the complete VPD at Offset 0x400.  The
>     abbreviated one only contains the PN, SN and EC Keywords, while the
>     complete VPD contains those plus various adapter constants contained in
>     V0, V1, etc.  And it also contains the Base Ethernet MAC Address in the
>     "NA" Keyword which the cxgb4 driver needs when it can't contact the
>     adapter firmware.  (We don't have the "NA" Keyword in the VPD Structure
>     at Offset 0x000 because that's not an allowed VPD Keyword in the PCI-E
>     3.0 specification.)
> 
>     Note that two other drivers look like they may also do something
>     similar, the Broadcom bnx2x and tg3.