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

From: Boris Ostrovsky
Date: Tue Sep 10 2019 - 13:48:30 EST

On 9/10/19 5:46 AM, Igor Druzhinin wrote:
> 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.

Right, I see it. Thanks for the explanation.

> 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.

Ah, yes. Multiple busses.

If that's the case then why don't we need to call xen_mcfg_late() for
the first device on each bus?