linux-kernel - Re: [PATCH v2] xen/pci: reserve MCFG areas earlier

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Message-ID: <76c350ed-9753-ef02-3813-2bf53b783227@oracle.com>
Date:   Fri, 13 Sep 2019 15:03:14 -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 v2] xen/pci: reserve MCFG areas earlier

On 9/12/19 2:31 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 in 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 as well. 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 register MCFG areas earlier
> upon the first invocation of xen_add_device(). It should be safe to do once
> since all the boot time buses must have their MCFG areas in MCFG table
> already and we don't support PCI bus hot-plug.
>
> Signed-off-by: Igor Druzhinin <igor.druzhinin@...rix.com>

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>

and applied to for-linus-5.4