Comments on: Enabling BIOS options on a live server with no rebooting

By: Rayne

Rayne — Wed, 14 Apr 2010 09:25:55 +0000

Great article!

Since this has to be done each time the server reboots, does it mean that the effects of doing this is not irreversible? For example, if I find that something isn't working right after doing this, I can simply restart my server and things will go back to the way it was?

By: Useful kernel and driver performance tweaks for your Linux server at time to bleed by Joe Damato

Tue, 28 Jul 2009 10:20:32 +0000

[...] Intel’s I/OAT also includes a feature called Direct Cache Access (DCA). DCA allows a driver to warm a CPU cache. A few NICs support DCA, the most popular (to my knowledge) is the Intel 10GbE driver (ixgbe). Refer to your NIC driver documentation to see if your NIC supports DCA. To enable DCA, a switch in the BIOS must be flipped. Some vendors supply machines that support DCA, but don’t expose a switch for DCA. If that is the case, see my last blog post for how to enable DCA manually. [...]

By: ice799

ice799 — Tue, 07 Jul 2009 02:40:18 +0000

Yep – but being that pedantic isn't particularly useful. I wanted to avoid spending time explaining MSRs and how they are used to keep my blog post short and to the point. Perhaps in a future blog post I will go into more detail here.

I thought that the first line of my article pointed out that I was describing a way to toggle CPU and chipset registers.

Thanks for reading.

By: ice799

ice799 — Tue, 07 Jul 2009 02:29:59 +0000

Yep, the driver could be patched to do this, too.

By: Wil

Wil — Tue, 07 Jul 2009 02:09:14 +0000

This seems like the sort of thing the driver should be capable of doing, either by default or via a command-line option.

By: MP

MP — Tue, 07 Jul 2009 01:26:48 +0000

First, congrats on the effort and solution. Nice work. There's one bit I'd like to be pedantic about, though: You're not enabling a BIOS option. You're enabling a CPU and chipset feature that, for one reason or another, you could not use the BIOS to enable (no access to BIOS, or the BIOS didn't expose that feature). It's true that the BIOS typically sets up CPU MSRs and PCI config space, although it's also quite common for modern OSes to make changes there as well. That's why you have the handy /dev/msr device for accessing those registers.

By: ice799

ice799 — Mon, 06 Jul 2009 23:38:37 +0000

This has to be done each time system reboots - but most people don't reboot their servers very often. If you need such a hack, you could add an init.d script which runs the hack and then loads the ioatdma module.

As far as VTx is concerned, the short answer is MAYBE.

The long answer is that there is a lock bit. If your vendor is particularly evil, their BIOS code will set the lock bit to OFF and not expose a switch for you to turn it back on. If this is the case, the only option you have is to flash your BIOS with a BIOS that does not set the lock bit to OFF.

Some vendors -don't- lock VTx off, but also don't expose a switch. If this is the case with your system, you can use a similar hack to the above to enable VTx. The VTx MSR lives at 0x3A and needs to be set to 5 for *each* CPU on the system.

If this is interesting enough, maybe I'll blog about it in the future.

Thanks for reading.

By: Piavlo

Piavlo — Mon, 06 Jul 2009 20:57:48 +0000

Will this work after a reboot or it has to bee done each time the OS boots?

Is ti possible to also enable Intel cpu virtualization (VTx) from user land since I have many types of boards with cpus supporting VTx but the board bioses do not expose enableing/disabling of this feature?

Alex