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

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Message-ID: <adefac87-c2b3-b67f-fb4d-d763ce920bef@citrix.com>
Date:   Sun, 8 Sep 2019 22:11:39 +0100
From:   Igor Druzhinin <igor.druzhinin@...rix.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        <xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
CC:     <jgross@...e.com>
Subject: Re: [PATCH] xen/pci: try to reserve MCFG areas earlier

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)
b) given (a) we cannot do it ourselves before acpi_init and after is too
late as we're already past ACPI PCI enumeration
c) we'd have to do it in the same place I call xen_mcfg_late() and it'd
be code duplication of what's already done by the existing code.

Igor