On Fri, 2019-08-09 at 09:06 +0200, Borislav Petkov wrote: > On Thu, Aug 08, 2019 at 08:54:17PM -0700, Isaku Yamahata wrote: > > Make PAT(Page Attribute Table) independent from > > MTRR(Memory Type Range Register). > > Some environments (mainly virtual ones) support only PAT, but not MTRR > > because PAT replaces MTRR. > > It's tricky and no gain to support both MTRR and PAT except compatibility. > > So some VM technologies don't support MTRR, but only PAT.
I do not think it is technically correct on bare metal. AFAIK, MTRR is still the only way to setup cache attribute in real-mode, which BIOS SMI handler relies on in SMM. > > This patch series makes PAT available on such environments without MTRR. > > And this "justification" is not even trying. Which "VM technologies" are > those? Why do we care? What's the impact? Why do we want this? > > You need to sell this properly. Agreed. If the situation is still the same, Xen does not support MTRR, and the kernel sets the PAT table to the BIOS hand-off state when MTRR is disabled. The change below accommodated the fact that Xen hypervisor enables WC before hand-off, which is different from the default BIOS hand-off state. The kernel does not support setting PAT when MTRR is disabled due to the dependency Isaku mentioned. https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1107094.html -Toshi
