Re: [Xen-devel] Xen: ARM: Support for mapping ECAM PCIe Config Space Specified In Static ACPI Table

[Jiandi An <anjiandi@xxxxxxxxxxxxxx>]

On 12/19/16 07:11, Julien Grall wrote:
> 
> 
> On 19/12/2016 13:20, Jaggi, Manish wrote:
>>> On 16/12/2016 15:49, Julien Grall wrote:
>>>> On 14/12/16 08:00, Jiandi An wrote:
>>>>> Xen currently doesn't map ECAM space specified in static ACPI table.
>>>>> Seeking opinion on how this should be handled properly.
>>>>> Each root complex ECAM region takes up 64K 4K pages (256MB).
>>>>> For some platforms there might be multiple root complexes.
>>>>> Is the plan to map all at once?Julien has mentioned support
>>>>> for mapping ECAM may come when support for PCI passthrough is
>>>>> added, is that right? What mechanism will it be? Will Xen or
>>>>> dom0 be the one that parses the staic ACPI tables and map the ECAM space?
>>>>
>>>> For performance reason, each ECAM region would need to be mapped at
>>>> once, so the stage-2 page table could take advantage of superpage (it
>>>> will mostly be 2MB).
>>>>
>>>> Now, I don't think Xen should map the ECAM region in stage-2 before
>>>> hand. All the regions may not be described in the MCFG and I would like
>>>> to see a generic solution.
>>>>
>>>> Looking at the code (see pci_create_ecam_create in drivers/pci/ecam.c),
>>>> ioremap is used. I believe the problem is the same for the 2 other
>>>> threads you sent ( [1] and [2]).
>>>>
>>>> So it might be good to look at hooking up a call to
>>>> XENMEM_add_to_physmap_range in ioremap.
>>>>
>>>> Any opinions?
>>>
>>> I thought a bit more about it and I realized we need to be cautious on
>>> how to proceed here.
>>>
>>> DOM0 will have a mix of real devices and emulated devices (e.g some part
>>> of the GIC). For the emulated devices, DOM0 should not call
>>> XENMEM_add_to_physmap_range. However, DOM0 is not aware what is emulated
>>> or not, so even the current approach (hooking up in platform device)
>>> seems fragile. We rely on Xen to say "this region cannot be mapped".
>>>
>>  Why not add support for parsing ACPI tables in Xen, from linux,  as we 
>> parse dt.
> 
> Because MMIO can be described in ASL too. I would rather avoid to have a 
> different behavior depending whether the MMIO has been described in static 
> table or ASL.
> 
> Cheers,
> 

I also think hooking up a call to XENMEM_add_to_physmap_range in ioremap
is not a good approach as ioremap() is commonly called in so many places.
It's not ideal to make a check of am I dom0 running under xen every time
ioremap() is called.  And Julien also pointed out, not every call to ioremap()
needs a stage 2 mapping.

-- 
Jiandi An
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux 
Foundation Collaborative Project.

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