Re: PCI Pass-through in Xen ARM - Draft 2.

[Julien Grall <julien.grall@citrix.com>]

Hi Stefano,

On 14/07/2015 18:46, Stefano Stabellini wrote:
>>> Linux provides a function (pci_for_each_dma_alias) which will return a
>>> requester ID for a given PCI device. It appears that the BDF (the 's' of sBDF
>>> is only internal to Linux and not part of the hardware) is equal to the
>>> requester ID on your platform but we can't assume it for anyone else.
>>
>> The PCI Express Base Specification states that the requester ID is "The
>> combination of a Requester's Bus Number, Device Number, and Function
>> Number that uniquely identifies the Requester."
>>
>> I think it is safe to assume BDF = requester ID on all platforms.
>
> With the catch that in case of ARI devices
> (http://pcisig.com/sites/default/files/specification_documents/ECN-alt-rid-interpretation-070604.pdf),
> BDF is actually BF because the device number is always 0 and the
> function number is 8 bits.

And some other problem such as broken PCI device...
Both Xen x86 (domain_context_mapping in drivers/passthrough/vtd/iommu.c) 
and Linux (pci_dma_for_each_alias) use a code more complex than 
requesterID = BDF.

So I don't think we can use requesterID = BDF in physdev op unless we 
are *stricly* sure this is valid.

Although, based on the x86 code, Xen should be able to translate the BDF 
into the requester ID...

Regards,

-- 
Julien Grall