linux-kernel - Re: Enabling RO on a VF

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Date:   Fri, 1 Oct 2021 11:59:15 +0000
From:   Haakon Bugge <haakon.bugge@...cle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
CC:     Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        OFED mailing list <linux-rdma@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Enabling RO on a VF



> On 1 Oct 2021, at 13:54, Jason Gunthorpe <jgg@...pe.ca> wrote:
> 
> On Fri, Oct 01, 2021 at 11:05:15AM +0000, Haakon Bugge wrote:
>> Hey,
>> 
>> 
>> Commit 1477d44ce47d ("RDMA/mlx5: Enable Relaxed Ordering by default
>> for kernel ULPs") uses pcie_relaxed_ordering_enabled() to check if
>> RO can be enabled. This function checks if the Enable Relaxed
>> Ordering bit in the Device Control register is set. However, on a
>> VF, this bit is RsvdP (Reserved for future RW
>> implementations. Register bits are read-only and must return zero
>> when read. Software must preserve the value read for writes to
>> bits.).
>> 
>> Hence, AFAICT, RO will not be enabled when using a VF.
>> 
>> How can that be fixed?
> 
> When qemu takes a VF and turns it into a PF in a VM it must emulate
> the RO bit and return one

I have a pass-through VF:

# lspci -s ff:00.0 -vvv
ff:00.0 Ethernet controller: Mellanox Technologies MT28800 Family [ConnectX-5 Ex Virtual Function]
[]
		DevCtl:	Report errors: Correctable- Non-Fatal- Fatal- Unsupported-
			RlxdOrd- ExtTag- PhantFunc- AuxPwr- NoSnoop- FLReset-


Håkon