Comment 4 for bug 1322526

It seems that IA32_MTRR_DEF_TYPE MSR can also serve a default:

The IA32_MTRR_DEF_TYPE MSR (named MTRRdefType MSR for the P6 family processors) sets the default properties of the regions of physical memory that are not encompassed by MTRRs. The functions of the flags and field in this register are as follows:

Type field, bits 0 through 7 - Indicates the default memory type used for those physical memory address ranges that do not have a memory type specified for them by an MTRR (see Table 11-8 for the encoding of this field). The legal values for this field are 0, 1, 4, 5, and 6. All other values result in a general-protection exception (#GP) being generated.

Intel recommends the use of the UC (uncached) memory type for all physical memory addresses where memory does not exist. To assign the UC type to nonexistent memory locations, it can either be specified as the default type in the Type field or be explicitly assigned with the fixed and variable MTRRs.