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Red Hat Enterprise Linux 5.6 - Message "kernel: Warning: pci_mmcfg_init marking xxxMB space uncacheable"

Issue

Environment:

Red Hat Enterprise Linux (RHEL) 5.6 (2.6.18-238.el5 x86_64)

DL360 G7

On above environment, the following warning message may be seen on boot:

kernel: Warning: pci_mmcfg_init marking 256MB space uncacheable.
MCFG table requires 64MB uncacheable only. Try booting with acpi_mcfg_max_pci_bus_num=on

This message means that kernel reserved 256MB uncacheable space, but MCFG table requires 64MB uncacheable only, (256-64=192)MB is useless space. As a solution, boot with acpi_mcfg_max_pci_bus_num=on.

The older kernel than kernel 2.6.18-238 dose not log this warning message.

On BZ581933, performance problem by memory shortage was reported. As the solution, this warning message was added on kernel 2.6.18-238.

581933 - pci_mmcfg_init() making some of main memory uncacheable:

Click here to access the information on "Red Hat Bugzilla – Bug 581933" Non-HPE site at https://bugzilla.redhat.com/show_bug.cgi\?id=581933.

NOTE: The above-mentioned URL will take you to a non-HP web site. HP does not control and is not responsible for information outside of the HP web site.

Click here to access the information on "Important: Red Hat Enterprise Linux 5.6 kernel security and bug fix update" Non-HPE site at http://rhn.redhat.com/errata/RHSA-2011-0017.html.

NOTE: The above-mentioned URL will take you to a non-HP web site. HP does not control and is not responsible for information outside of the HP web site.

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Solution

Add acpi_mcfg_max_pci_bus_num=on on /boot/grub/grub.conf and reboot.

Example:

kernel /vmlinuz-2.6.18-238.el5 ro root=LABEL=/ rhgb

quiet acpi_mcfg_max_pci_bus_num=on

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Document title: Red Hat Enterprise Linux 5.6 - Message "kernel: Warning: pci_mmcfg_init marking xxxMB space uncacheable"
Document ID: emr_na-c02820254-1
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