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

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Message-ID: <9ac1f34b-ea2a-3818-4cbd-a22a9a475dd4@oracle.com>
Date:   Tue, 10 Sep 2019 17:19:45 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Igor Druzhinin <igor.druzhinin@...rix.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     jgross@...e.com
Subject: Re: [Xen-devel] [PATCH] xen/pci: try to reserve MCFG areas earlier

On 9/10/19 4:36 PM, Igor Druzhinin wrote:
> On 10/09/2019 18:48, Boris Ostrovsky wrote:
>> 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:
>>>>>
>>>>>> 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?
>>
> Ideally, yes - we'd like to call it for every bus discovered. But boot
> time buses are already in MCFG (otherwise system boot might not simply
> work as Jan pointed out) so it's not strictly required. The only case is
> a potential PCI bus hot-plug but I'm not sure it actually works in
> practice and we certainly didn't support it before. It might be solved
> theoretically by subscribing to acpi_bus_type that is available after
> acpi_init().

OK. Then *I think* we can drop late_initcall() but I would really like
to hear when others think.

-boris