Re: [Xen-devel] [PATCH] xen/pci: try to reserve MCFG areas earlier

[Igor Druzhinin]

On 10/09/2019 02:47, Boris Ostrovsky wrote:
> On 9/9/19 5:48 PM, Igor Druzhinin wrote:
>> On 09/09/2019 20:19, Boris Ostrovsky wrote:
>>> On 9/8/19 7:37 PM, Igor Druzhinin wrote:
>>>> On 09/09/2019 00:30, Boris Ostrovsky wrote:
>>>>> On 9/8/19 5:11 PM, Igor Druzhinin wrote:
>>>>>> On 08/09/2019 19:28, Boris Ostrovsky wrote:
>>>>>>> Would it be possible for us to parse MCFG ourselves in pci_xen_init()? I
>>>>>>> realize that we'd be doing this twice (or maybe even three times since
>>>>>>> apparently both pci_arch_init()Â and acpi_ini() do it).
>>>>>>>
>>>>>> I don't thine it makes sense:
>>>>>> a) it needs to be done after ACPI is initialized since we need to parse
>>>>>> it to figure out the exact reserved region - that's why it's currently
>>>>>> done in acpi_init() (see commit message for the reasons why)
>>>>> Hmm... We should be able to parse ACPI tables by the time
>>>>> pci_arch_init() is called. In fact, if you look at
>>>>> pci_mmcfg_early_init() you will see that it does just that.
>>>>>
>>>> The point is not to parse MCFG after acpi_init but to parse DSDT for
>>>> reserved resource which could be done only after ACPI initialization.
>>> OK, I think I understand now what you are trying to do --- you are
>>> essentially trying to account for the range inserted by
>>> setup_mcfg_map(), right?
>>>
>> Actually, pci_mmcfg_late_init() that's called out of acpi_init() -
>> that's where MCFG areas are properly sized. 
> 
> pci_mmcfg_late_init() reads the (static) MCFG, which doesn't need DSDT parsing, does it? setup_mcfg_map() OTOH does need it as it uses data from _CBA (or is it _CRS?), and I think that's why we can't parse MCFG prior to acpi_init(). So what I said above indeed won't work.
> 

No, it uses is_acpi_reserved() (it's called indirectly so might be well
hidden) to parse DSDT to find a reserved resource in it and size MCFG
area accordingly. setup_mcfg_map() is called for every root bus
discovered and indeed tries to evaluate _CBA but at this point
pci_mmcfg_late_init() has already finished MCFG registration for every
cold-plugged bus (which information is described in MCFG table) so those
calls are dummy.

>> setup_mcfg_map() is mostly
>> for bus hotplug where MCFG area is discovered by evaluating _CBA method;
>> for cold-plugged buses it just confirms that MCFG area is already
>> registered because it is mandated for them to be in MCFG table at boot time.
>>
>>> The other question I have is why you think it's worth keeping
>>> xen_mcfg_late() as a late initcall. How could MCFG info be updated
>>> between acpi_init() and late_initcalls being run? I'd think it can only
>>> happen when a new device is hotplugged.
>>>
>> It was a precaution against setup_mcfg_map() calls that might add new
>> areas that are not in MCFG table but for some reason have _CBA method.
>> It's obviously a "firmware is broken" scenario so I don't have strong
>> feelings to keep it here. Will prefer to remove in v2 if you want.
> 
> Isn't setup_mcfg_map() called before the first xen_add_device() which is where you are calling xen_mcfg_late()?
> 

setup_mcfg_map() calls are done in order of root bus discovery which
happens *after* the previous root bus has been enumerated. So the order
is: call setup_mcfg_map() for root bus 0, find that
pci_mmcfg_late_init() has finished MCFG area registration, perform PCI
enumeration of bus 0, call xen_add_device() for every device there, call
setup_mcfg_map() for root bus X, etc.

Igor