On Fri, 2015-06-12 at 08:59 +0100, Jan Beulich wrote: > >>> On 12.06.15 at 01:23, <toshi.k...@hp.com> wrote: > > There are two usages on MTRRs: > > 1) MTRR entries set by firmware > > 2) MTRR entries set by OS drivers > > > > We can obsolete 2), but we have no control over 1). As UEFI firmwares > > also set this up, this usage will continue to stay. So, we should not > > get rid of the MTRR code that looks up the MTRR entries, while we have > > no need to modify them. > > > > Such MTRR entries provide safe guard to /dev/mem, which allows > > privileged user to access a range that may require UC mapping while > > the /dev/mem driver blindly maps it with WB. MTRRs converts WB to UC in > > such a case. > > But it wouldn't be impossible to simply read the MTRRs upon boot, > store the information, disable MTRRs, and correctly use PAT to > achieve the same effect (i.e. the "blindly maps" part of course > would need fixing).
It could be done, but I do not see much benefit of doing it. One of the reasons platform vendors set MTRRs is so that a system won't hit a machine check when an OS bug leads an access with a wrong cache type. A machine check is hard to analyze and can be seen as a hardware issue by customers. Emulating MTRRs with PAT won't protect from such a bug. > > UEFI memory table has memory attribute, which describes cache types > > supported in physical memory ranges. However, this information gets > > lost when it it is converted to e820 table. > > I'm afraid you rather don't want to trust that information, as > firmware vendors frequently screw it up. Could be, but we need to use firmware info when necessary... Thanks, -Toshi -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
