linux-kernel - Re: Enabling RO on a VF

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Message-ID: <20211001115455.GJ3544071@ziepe.ca>
Date:   Fri, 1 Oct 2021 08:54:55 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Haakon Bugge <haakon.bugge@...cle.com>
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 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

Jason