Re: [PATCH] vfio/listener: Only warn on misalignment for VFIO-owned regions

[Alex Williamson <alex@shazbot.org>]

On Mon, 6 Jul 2026 17:12:07 +0200
Cédric Le Goater <clg@redhat.com> wrote:

> On 7/6/26 16:20, Alex Williamson wrote:
> > On Thu,  2 Jul 2026 18:06:40 +0200
> > Cédric Le Goater <clg@redhat.com> wrote:
> >   
> >> Any device on the board can have non-page-aligned memory regions. The
> >> VFIO listener should not warn about misalignment for regions that are
> >> not VFIO-related.
> >>
> >> Replace the vfio_known_safe_misalignment() whitelist with a
> >> vfio_get_vfio_device() ownership check: only warn when a VFIO device's
> >> own region is misaligned. For all other regions, emit a trace event
> >> and silently skip. Extract the misalignment test into a
> >> vfio_section_misaligned() helper for readability.
> >>
> >> Note: vfio_get_vfio_device() currently only covers VFIO PCI devices,
> >> including vfio-user-pci.  Other VFIO backends (AP, CCW) don't expose
> >> memory regions through this path so this is sufficient for now.  
> > 
> > I thought the whole point was to warn when there are non-vfio devices
> > that we cannot insert into the DMA map.  851d6d1a0ff2 added tpm-crb-cmd
> > to an ignore list because it's not a DMA target.  The intention of that
> > ignore list was to continue to evaluate devices that are misaligned
> > and either add them to the ignore list or determine they could be a
> > valid DMA target and correct the alignment.  
> 
> If the goal was to use the VFIO listener to catch all misaligned regions,
> and potentially fix them, we should reconsider that approach :
>   
>    https://lore.kernel.org/qemu-devel/20260701080843.2418461-1-clg@redhat.com/

Arguably ok, likely no tpm device is a useful DMA target.
 
> As Peter pointed out, regions that are not page-aligned regions are valid :
> 
>    https://lore.kernel.org/qemu-devel/CAFEAcA-QHJ=HaBc5WSLLap8NdDw+tty4EFfGt6cmmNOK6e_m7w@mail.gmail.com/

Yes, they are valid, no argument.  We're trying to DMA map things.  We
can't DMA map things that aren't page-aligned and sized.  If it's not
likely a DMA target, well, that's the purpose of the ignore list.  If
it might be a DMA target, this is intended to log why the VM may not
work correctly vs bare metal.
 
> and aligning them would require a machine compat for migration.
> Not addressed in :
> 
>    https://lore.kernel.org/qemu-devel/20260630173245.2070268-1-clg@redhat.com/

Which I think highlights why we might rather see warnings than bury the
issue in traces.

> > Can this even legitimately trigger with a vfio-pci device?    
> 
> This would be a real error to report. Right ?

I suspect it's not possible, so as proposed here we're turning a blind
eye to the issue we intended to monitor and validate, and we replace it
with likely dead code.  Thanks,

Alex