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Subject: Re: [Xen-devel] [PATCH] xen/pci: try to reserve MCFG areas earlier
To:     Igor Druzhinin <igor.druzhinin@citrix.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Cc:     jgross@suse.com
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From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <43e492ff-f967-7218-65c4-d16581fabea3@oracle.com>
Date:   Mon, 9 Sep 2019 21:47:26 -0400
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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:
>>>>>> On 9/6/19 7:00 PM, Igor Druzhinin wrote:
>>>>>>> On 06/09/2019 23:30, Boris Ostrovsky wrote:
>>>>>>>> Where is MCFG parsed? pci_arch_init()?
>>>>>>>>> It happens twice:
>>>>>>> 1) first time early one in pci_arch_init() that is arch_initcall - that
>>>>>>> time pci_mmcfg_list will be freed immediately there because MCFG area is
>>>>>>> not reserved in E820;
>>>>>>> 2) second time late one in acpi_init() which is subsystem_initcall right
>>>>>>> before where PCI enumeration starts - this time ACPI tables will be
>>>>>>> checked for a reserved resource and pci_mmcfg_list will be finally
>>>>>>> populated.
>>>>>>>
>>>>>>> The problem is that on a system that doesn't have MCFG area reserved in
>>>>>>> E820 pci_mmcfg_list is empty before acpi_init() and our PCI hooks are
>>>>>>> called in the same place. So MCFG is still not in use by Xen at this
>>>>>>> point since we haven't reached our xen_mcfg_late().
>>>>>> 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()  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.

> 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()?


-boris