Re: [Xen-devel] [PATCH 2/5] iommu / x86: move call to scan_pci_devices() out of vendor code

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 13 May 2019 08:36
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: Brian Woods <brian.woods@xxxxxxx>; Suravee Suthikulpanit 
> <suravee.suthikulpanit@xxxxxxx>; Andrew
> Cooper <Andrew.Cooper3@xxxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>; 
> Wei Liu
> <wei.liu2@xxxxxxxxxx>; Kevin Tian <kevin.tian@xxxxxxxxx>; xen-devel 
> <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Subject: Re: [PATCH 2/5] iommu / x86: move call to scan_pci_devices() out of 
> vendor code
> 
> >>> On 08.05.19 at 15:24, <paul.durrant@xxxxxxxxxx> wrote:
> > It's not vendor specific so it shouldn't really be there.
> 
> Perhaps, but this needs better justification:
> 
> > --- a/xen/drivers/passthrough/vtd/iommu.c
> > +++ b/xen/drivers/passthrough/vtd/iommu.c
> > @@ -2372,10 +2372,6 @@ static int __init vtd_setup(void)
> >      P(iommu_hap_pt_share, "Shared EPT tables");
> >  #undef P
> >
> > -    ret = scan_pci_devices();
> > -    if ( ret )
> > -        goto error;
> > -
> >      ret = init_vtd_hw();
> 
> Even after some looking around, it's not obvious to me that the latter
> call doesn't depend on PCI devices being known, more specifically
> segment 0's bus2bridge[] having been filled. Nor can I tell whether
> there would be some noticeable misbehavior (prior to any guests
> starting) if there was a dependency and it got broken by the re-
> ordering.

I don't see any dependency but the code is somewhat tangled. Perhaps it would 
be better to build the PCI topology *before* IOMMU init and then iterate over 
the the devices after init to do the group assignment. I certainly can't see 
anything in the scan as it stands that would need the IOMMU to have been 
initialized.

  Paul

> 
> Jan
> 


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