Change in coreboot[master]: cpu/x86/mtrr: Add check_var_mtrr_range_enabled() helper function

[Arthur Heymans (Code Review)]

Attention is currently required from: Furquan Shaikh, Tim Wawrzynczak, Subrata Banik, Angel Pons.
Arthur Heymans has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/56151 )

Change subject: cpu/x86/mtrr: Add check_var_mtrr_range_enabled() helper function
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Patch Set 1:

(2 comments)

File src/cpu/x86/mtrr/earlymtrr.c:

https://review.coreboot.org/c/coreboot/+/56151/comment/f2f455c7_b620e728 
PS1, Line 136: static bool get_var_mtrr_range(int index, uintptr_t *base_address, size_t *length)
             : {
             : 	bool ret = false;
             : 	uint32_t address_bits;
             : 	uint64_t address_mask;
             : 	const uint32_t msr_reg = MTRR_PHYS_BASE(index);
             : 	uint64_t mask;
             : 	union {
             : 		uint64_t u64;
             : 		msr_t s;
             : 	} msr_a;
             : 	union {
             : 		uint64_t u64;
             : 		msr_t s;
             : 	} msr_m;
             : 
             : 	address_bits = cpu_phys_address_size();
             : 	address_mask = (1ULL << address_bits) - 1;
             : 
             : 	msr_a.s = rdmsr(msr_reg);
             : 	msr_m.s = rdmsr(msr_reg + 1);
             : 	if (msr_m.u64 & MTRR_PHYS_MASK_VALID) {
             : 		*base_address = (msr_a.u64 & 0xfffffffffffff000ULL)
             : 				& address_mask;
             : 
             : 		mask = (msr_m.u64 & 0xfffffffffffff000ULL) & address_mask;
             : 		*length = (~mask & address_mask) + 1;
             : 		ret = true;
             : 	}
             : 	return ret;
             : }
maybe use struct region?

https://review.coreboot.org/c/coreboot/+/56151/comment/f2a9b5f0_431473b8 
PS1, Line 168: bool check_var_mtrr_range_enabled(uintptr_t base, size_t size)
This function relies on the caller to align base and size of the address range you want to check. That's not a good API.
I'd prefer a function that can check if a certain arbitrary range is cached a certain way: UC/RW/RO/WC...
e.g.: checking if [0xff700000, 0xffffffff] is covered by RO caching type MTRR.
You also don't want to make assumptions on the MTRR default caching type: read it out first.