On Fri, Aug 09, 2019 at 07:51:17PM +0000, "Kani, Toshi" <toshi.k...@hpe.com> wrote:
> 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. Then you're claiming if it's baremetal, both MTRR and PAT should be enabled/disabled at the same time? > > > 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 Thanks for supplement. In addition to Xen, KVM+qemu can enable/disable MTRR, PAT independently. So user may want to disable MTRR to reduce attack surface. ACRN doesn't support MTRR. Let me include those description for next respin. -- Isaku Yamahata <isaku.yamah...@gmail.com>
