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

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Date:   Tue, 10 Sep 2019 11:08:59 +0100
From:   Igor Druzhinin <igor.druzhinin@...rix.com>
To:     Jan Beulich <jbeulich@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        <linux-kernel@...r.kernel.org>
CC:     <xen-devel@...ts.xenproject.org>, Juergen Gross <jgross@...e.com>
Subject: Re: [Xen-devel] [PATCH] xen/pci: try to reserve MCFG areas earlier

On 10/09/2019 10:55, Jan Beulich wrote:
> On 10.09.2019 11:46, Igor Druzhinin wrote:
>> On 10/09/2019 02:47, Boris Ostrovsky wrote:
>>> On 9/9/19 5:48 PM, Igor Druzhinin wrote:
>>>> 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.
>>>
>>
>> No, it uses is_acpi_reserved() (it's called indirectly so might be well
>> hidden) to parse DSDT to find a reserved resource in it and size MCFG
>> area accordingly. setup_mcfg_map() is called for every root bus
>> discovered and indeed tries to evaluate _CBA but at this point
>> pci_mmcfg_late_init() has already finished MCFG registration for every
>> cold-plugged bus (which information is described in MCFG table) so those
>> calls are dummy.
> 
> I don't think they're strictly dummy. Even for boot time available devices
> iirc there's no strict requirement for there to be respective data in MCFG.
> Such a requirement exists only for devices which are actually needed to
> start the OS (disk or network, perhaps video or alike), or maybe even just
> its loader.
> 

This was my interpretation of 4.1.3 of "PCI Frimware specification":
"Memory mapped configuration base addresses for non-hot pluggable host
bridges must be described using MCFG table." Although, I admit that
"non-hot pluggable" might mean available at boot as well.

Igor