linux-kernel - Re: [PATCH] iommu: intel: remove flooding of non-error logs, when new-DMA-PTE is the same as old-DMA-PTE.

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Date:   Mon, 11 Oct 2021 08:52:47 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Ajay Garg <ajaygargnsit@...il.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] iommu: intel: remove flooding of non-error logs, when
 new-DMA-PTE is the same as old-DMA-PTE.

On Mon, 11 Oct 2021 11:49:33 +0530
Ajay Garg <ajaygargnsit@...il.com> wrote:

> The flooding was seen today again, after I booted the host-machine in
> the morning.
> Need to look what the heck is going on ...
> 
> On Sun, Oct 10, 2021 at 11:45 AM Ajay Garg <ajaygargnsit@...il.com> wrote:
> >  
> > > I'll try and backtrack to the userspace process that is sending these ioctls.
> > >  
> >
> > The userspace process is qemu.
> >
> > I compiled qemu from latest source, installed via "sudo make install"
> > on host-machine, rebooted the host-machine, and booted up the
> > guest-machine on the host-machine. Now, no kernel-flooding is seen on
> > the host-machine.
> >
> > For me, the issue is thus closed-invalid; admins may take the
> > necessary action to officially mark ;)

Even this QEMU explanation doesn't make a lot of sense, vfio tracks
userspace mappings and will return an -EEXIST error for duplicate or
overlapping IOVA entries.  We expect to have an entirely empty IOMMU
domain when a device is assigned, but it seems the only way userspace
can trigger duplicate PTEs would be if mappings already exist, or we
have a bug somewhere.

If the most recent instance is purely on bare metal, then it seems the
host itself has conflicting mappings.  I can only speculate with the
limited data presented, but I'm suspicious there's something happening
with RMRRs here (but that should also entirely preclude assignment).
dmesg, lspci -vvv, and VM configuration would be useful.  Thanks,

Alex