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

From: "Jan Beulich" <JBeulich@suse.com>
To: Paul Durrant <paul.durrant@citrix.com>
Cc: Kevin Tian <kevin.tian@intel.com>, Wei Liu <wei.liu2@citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Brian Woods <brian.woods@amd.com>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [PATCH 2/5] iommu / x86: move call to scan_pci_devices() out of vendor code
Date: Wed, 15 May 2019 00:29:39 -0600	[thread overview]
Message-ID: <5CDBB1D3020000780022F109@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <9a476c95c1fb410b9330c45fa6799b9a@AMSPEX02CL03.citrite.net>

>>> On 14.05.19 at 18:13, <Paul.Durrant@citrix.com> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@suse.com]
>> Sent: 13 May 2019 08:36
>> To: Paul Durrant <Paul.Durrant@citrix.com>
>> Cc: Brian Woods <brian.woods@amd.com>; Suravee Suthikulpanit 
> <suravee.suthikulpanit@amd.com>; Andrew
>> Cooper <Andrew.Cooper3@citrix.com>; Roger Pau Monne <roger.pau@citrix.com>; Wei 
> Liu
>> <wei.liu2@citrix.com>; Kevin Tian <kevin.tian@intel.com>; xen-devel 
> <xen-devel@lists.xenproject.org>
>> 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@citrix.com> 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.

Ah, yes, that's likely a better model. As to the dependency
aspect: pci_add_device() calls iommu_add_device(). But oddly
enough _scan_pci_devices() calls alloc_pdev(), not
pci_add_device(). So indeed there doesn't look to be any
dependency at present.

Jan

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