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

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Message-ID: <1695c88d-e5ad-1854-cdef-3cd95c812574@oracle.com>
Date:   Sun, 8 Sep 2019 19:30:41 -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: [PATCH] xen/pci: try to reserve MCFG areas earlier

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.

> 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.


If we manage to parse MCFG ourselves early then maybe we won't not need
xen_mcfg_late()? We can call PHYSDEVOP_pci_mmcfg_reserved right away.

-boris