Re: [Xen-devel] [PATCH 1/2] VT-d: don't pass bridge devices to domain_context_mapping_one()

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Tuesday, January 7, 2020 3:38 PM
> 
> When passed a non-NULL pdev, the function does an owner check when it
> finds an already existing context mapping. Bridges, however, don't get
> passed through to guests, and hence their owner is always going to be
> Dom0, leading to the assigment of all but one of the function of multi-
> function PCI devices behind bridges to fail.
> 
> Reported-by: Marek Marczykowski-Górecki
> <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Note: This was reported as an apparent regression from XSA-302 / -306.
>       So far I haven't been able to figure out how the code would have
>       worked before, i.e. to me it looks like a pre-existing problem.
>       This leaves the risk of the change here papering over another
>       issue.
> 
> --- a/xen/drivers/passthrough/vtd/iommu.c
> +++ b/xen/drivers/passthrough/vtd/iommu.c
> @@ -1499,7 +1499,7 @@ static int domain_context_mapping(struct
>              break;
> 
>          ret = domain_context_mapping_one(domain, drhd->iommu, bus, devfn,
> -                                         pci_get_pdev(seg, bus, devfn));
> +                                         NULL);

the relationship between NULL and a bridge device is not obvious
by just looking at the code. Can you at least add some comment
here, or possibly be clearer by having a mapping_bridge wrapper?

> 
>          /*
>           * Devices behind PCIe-to-PCI/PCIx bridge may generate different
> @@ -1509,7 +1509,7 @@ static int domain_context_mapping(struct
>          if ( !ret && pdev_type(seg, bus, devfn) == DEV_TYPE_PCIe2PCI_BRIDGE
> &&
>               (secbus != pdev->bus || pdev->devfn != 0) )
>              ret = domain_context_mapping_one(domain, drhd->iommu, secbus,
> 0,
> -                                             pci_get_pdev(seg, secbus, 0));
> +                                             NULL);
> 
>          break;
> 

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