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

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Message-ID: <5054ad91-5b87-652c-873a-b31758948bd7@oracle.com>
Date:   Fri, 6 Sep 2019 18:30:21 -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 9/3/19 8:20 PM, Igor Druzhinin wrote:
> If MCFG area is not reserved in E820, Xen by default will defer its usage
> until Dom0 registers it explicitly after ACPI parser recognizes it as
> a reserved resource in DSDT. Having it reserved in E820 is not
> mandatory according to "PCI Firmware Specification, rev 3.2" (par. 4.1.2)
> and firmware is free to keep a hole E820 in that place. Xen doesn't know
> what exactly is inside this hole since it lacks full ACPI view of the
> platform therefore it's potentially harmful to access MCFG region
> without additional checks as some machines are known to provide
> inconsistent information on the size of the region.
>
> Now xen_mcfg_late() runs after acpi_init() which is too late as some basic
> PCI enumeration starts exactly there. Trying to register a device prior
> to MCFG reservation causes multiple problems with PCIe extended
> capability initializations in Xen (e.g. SR-IOV VF BAR sizing). There are
> no convenient hooks for us to subscribe to so try to register MCFG
> areas earlier upon the first invocation of xen_add_device(). 


Where is MCFG parsed? pci_arch_init()?

-boris


> Keep the
> existing initcall in case information of MCFG areas is updated later
> in acpi_init().
>