Re: [Qemu-devel] [PATCH v2] intel_iommu: check misordered init when realize

[Peter Xu]

On Fri, Feb 24, 2017 at 12:07:33AM -0500, Pankaj Gupta wrote:
> Hello Peter,

Hi, Pankaj!

> 
> This solution looks to check dependency of 'vfio-pci' over 'intel-iommu'
> before 'intel-iommu' is not initialized.
> 
> Overall it looks good to me, just a small nit below.
>  
> > 
> > Intel vIOMMU devices are created with "-device" parameter, while here
> > actually we need to make sure the dmar device be created before other
> > PCI devices (like vfio-pci) so that we know iommu_fn will be setup
> > correctly before realizations of those PCI devices (it is sensible that
> > PCI device fetch these info during its realization). Now this ordering
> > yet cannot be achieved elsewhere, and devices will be created in the
> > order that user specified. That might be dangerous.
> > 
> > Here we add one more function to detect this kind of misordering issue,
> > then report to guest. Currently, the only known device that is affected
> > by this VT-d defect is the vfio-pci typed devices. So for now we just
> > check against it to make sure we are safe.
> > 
> > Signed-off-by: Peter Xu <address@hidden>
> > ---
> >  hw/i386/intel_iommu.c | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> > 
> > diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> > index 22d8226..b723ece 100644
> > --- a/hw/i386/intel_iommu.c
> > +++ b/hw/i386/intel_iommu.c
> > @@ -2560,6 +2560,24 @@ static bool vtd_decide_config(IntelIOMMUState *s,
> > Error **errp)
> >      return true;
> >  }
> >  
> > +/*
> > + * TODO: we should have a better way to achieve the ordering rather
> > + * than this misorder check explicitly against vfio-pci. After all, no
> > + * one should be blamed for this, and vfio-pci did nothing wrong.
> > + */
> > +static bool vtd_detected_misorder_init(Error **errp)
> > +{
> > +    Object *dev = object_resolve_path_type("", "vfio-pci", NULL);
> > +
> > +    if (dev) {
> > +        error_setg(errp, "Please specify \"intel-iommu\" before all the 
> > rest
> 
>             "before all the rest" does not give much clue to user. Do you 
> think better
>             error message would help? just a thought.

Yes this is my intention to emphasize that we should possibly put
intel-iommu even before all the PCI devices. As mentioned above (and
also in the commit message), although vfio-pci is the only one that is
affected by this, we should probably put intel-iommu even before all
the rest of PCI devices. E.g., in the future we can have new devices
that need this dependency as well (that vt-d being inited before that
device), which is still legal imho.

Meanwhile, I intended to avoid naming "vfio-pci" here since it'll let
user think of "something bad with vfio-pci" but actually vfio-pci did
nothing wrong.

Thanks,

-- peterx