Re: [PATCH 12/12] xen/mtrr: Add mtrr_if support for Xen mtrr

Jeremy Fitzhardinge
2010-09-28T18:58:09Z

On 09/28/2010 11:46 AM, H. Peter Anvin wrote:
> Well, we're specifically talking about a virtual machine which has
> direct access to hardware, so it is concerned about the real physical
> memory properties of real physical pages. If we can assume that
> BIOS/Xen will always set up MTRR correctly then there shouldn't be any
> need for the kernel to modify the MTRR itself. How true is that in
> general? I don't know, but if we could rely on BIOS then there'd never
> be a need to touch MTRR, would there?
> Well, in the past MTRRs were abused for device properties mainly by the
> X server, but other than that, no, not really. The other thing we do is
> the MTRR cleanup (which doesn't involve /proc/mtrr) to deal with
> brokenness in the BIOS setup, but that really belongs in the hypervisor
> in your case since it fundamentally affects how memory is handled.

Yeah, the hypervisor should definitely deal with that. I have no
problem in principle with leaving MTRRs entirely to Xen, but I was just
concerned about possible repercussions. Certainly when I first did this
work, I was using Fedora 8 whose X server did depend on /proc/mtrr for
good performance.

J