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

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Message-ID: <4d3bf854-51de-99e4-9a40-a64c581bdd10@citrix.com>
Date:   Mon, 9 Sep 2019 00:37:12 +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 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.

>> 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.

Again, this cannot be done untile acpi_init finishes basic setup to
parse DSDT.

Igor