fix(x86_64): specify IA32_MTRRdefType MSR on boot

The IA32_MTRRdefType MSR describes the default memory type
used for physical addresses which are outside of any configured
memory ranges, as well as whether MTRRs and fixed ranges
are enabled.

Without enabling this MSR, guest kernel is not able to change
memory types for memory ranges outside main memory. This was
causing pmem region to be marked as `uncached-minus` which
instructed the cpu to skip all caches (L1, L2, L3) when
reading/writing to that memory region.

To fix this set IA32_MTRRdefType set to 0x806:
- bit 11 enables MTRR
- bits 0..7 specify the memory type used by default
  - value 6 represents `write-back` type

Now guest can correctly set pmem memory region to
`write-back` memory type.

Signed-off-by: Egor Lazarchuk <[email protected]>

Author: ShadowCurse

src/vmm/src/arch/x86_64/msr.rs

@@ -419,6 +419,15 @@ pub fn create_boot_msr_entries() -> Vec<kvm_msr_entry> {
             data: u64::from(MSR_IA32_MISC_ENABLE_FAST_STRING),
             ..Default::default()
         },
+        // set default memory type for physical memory outside configured
+        // memory ranges to write-back by setting MTRR enable bit and
+        // setting memory type to write-back (value 6).
+        // https://wiki.osdev.org/MTRR
+        kvm_msr_entry {
+            index: MSR_MTRRdefType,
+            data: 0x806,
+            ..Default::default()
+        },
     ]
 }