Re: [Xen-devel] MTRR on Xen - BIOS use and implications for Linux
From: Luis R. Rodriguez
Date: Tue Mar 29 2016 - 18:28:23 EST
On Tue, Mar 29, 2016 at 04:14:41PM -0600, Toshi Kani wrote:
> On Tue, 2016-03-29 at 10:22 -0700, Luis R. Rodriguez wrote:
> > On Thu, Mar 17, 2016 at 11:56 AM, Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
> > wrote:
> > > On Thu, Mar 17, 2016 at 11:13:03AM +0000, David Vrabel wrote:
> > > > On 16/03/16 20:08, Luis R. Rodriguez wrote:
> > > > > Toshi noted a while ago as well that if BIOS/firmware enables MTRR
> > > > > but the kernel does not have it enabled one issue might have been
> > > > > any MTRRs set up by the BIOS and ensuring the mapping is respected,
> > To be clear the requirement expressed here was needing at least to
> > implement get_mtrr() on the Linux Xen guest side, it'd call the
> > already implemented hypercall XENPF_read_memtype in turn. Toshi had
> > hinted this was perhaps needed on the Linux Xen guest side given that
> > the BIOS may have set up MTRRs on its own, so we needed the guest to
> > be able to get the right type for a particular range. More on that
> > below
> No, I did not say that. :-) I said it because you mentioned that you
> wanted to disable MTRRs even though BIOS enabled MTRRs.
> My point is that the kernel should not change the MTRR setup. That is:
> - If BIOS enables MTRRs, then kernel needs to keep them enabled.
> - If BIOS disables MTRRs, then the kernel needs to keep them disabled.
> Therefore, the kernel can keep MTRRs disabled on Xen guests.
Indeed, thanks for the clarification. That knocks this last concern
out of the way. The remaining aspects then are simply optimizations