x86: mtrr: Constrain WB MTRR to max phys mem prior to cleanup

If BIOS maps all or most of the phys address space as write-back, regardless of the amount of actual RAM, then mtrr_cleanup may create superfluous MTRR entries, or may be unable to create an optimal map (or may use all MTRRs attempting to do so, thus making cleanup pointless). Ideally, WB MTRRs should not be used for physical memory addresses where memory does not exist (Intel SDM, Vol 3A, 11.11.2.1).

For example, Dell Precision T5400 wkstn w/ rev. A04 BIOS constructs the following MTRR map for 10GB memory:

   kernel: [    0.000000] MTRR variable ranges enabled:
   kernel: [    0.000000]   0 base 0000000000 mask 2000000000 write-back
   kernel: [    0.000000]   1 base 00CFF00000 mask 3FFFF00000 uncachable
   kernel: [    0.000000]   2 base 00D0000000 mask 3FF0000000 uncachable
   kernel: [    0.000000]   3 base 00E0000000 mask 3FE0000000 uncachable
   kernel: [    0.000000]   4 base 1FF0000000 mask 3FF0000000 uncachable
   kernel: [    0.000000]   5 disabled
   kernel: [    0.000000]   6 disabled
   kernel: [    0.000000]   7 disabled
   kernel: [    0.000000] x86 PAT enabled: cpu 0 ......
   kernel: [    0.000000] original variable MTRRs
   kernel: [    0.000000] reg 0, base: 0GB, range: 128GB, type WB
   kernel: [    0.000000] reg 1, base: 3327MB, range: 1MB, type UC
   kernel: [    0.000000] reg 2, base: 3328MB, range: 256MB, type UC
   kernel: [    0.000000] reg 3, base: 3584MB, range: 512MB, type UC
   kernel: [    0.000000] reg 4, base: 130816MB, range: 256MB, type UC
   kernel: [    0.000000] total RAM covered: 130047M

mtrr_cleanup cannot construct an optimal map for this configuration. Additionally, if the mtrr_gran_size/mtrr_chunk_size is specified, mtrr_cleanup will use all entries attempting to map the full 128GB phys address range.

If a WB MTRR entry which maps phys addr range [0, > max_pfn) exists, constrain the entry to [0, roundup_pow_of_two(max_pfn) ) prior to generating potential solutions. The example MTRR map is optimized to:

   kernel: [    0.000000] New variable MTRRs
   kernel: [    0.000000] reg 0, base: 0GB, range: 2GB, type WB
   kernel: [    0.000000] reg 1, base: 2GB, range: 1GB, type WB
   kernel: [    0.000000] reg 2, base: 3GB, range: 256MB, type WB
   kernel: [    0.000000] reg 3, base: 3327MB, range: 1MB, type UC
   kernel: [    0.000000] reg 4, base: 4GB, range: 4GB, type WB
   kernel: [    0.000000] reg 5, base: 8GB, range: 8GB, type WB

---
 arch/x86/kernel/cpu/mtrr/cleanup.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

index bdda2e6..ee399c3 100644
@@ -714,6 +714,16 @@ int __init mtrr_cleanup(unsigned address_bits)
 	if (mtrr_tom2)
 		x_remove_size = (mtrr_tom2 >> PAGE_SHIFT) - x_remove_base;
 
+	/* Constrain a WB entry that maps entire phys addr space to max mem */
+	for (i = 0; i < num_var_ranges; i++) {
+		if (range_state[i].type == MTRR_TYPE_WRBACK &&
+		    range_state[i].base_pfn == 0 &&
+		    range_state[i].size_pfn > max_pfn) {
+			range_state[i].size_pfn = roundup_pow_of_two(max_pfn);
+			break;
+		}
+	}
+
 	nr_range = x86_get_mtrr_mem_range(range, 0, x_remove_base, x_remove_size);
 	/*
 	 * [0, 1M) should always be covered by var mtrr with WB

1.7.5.4