soc/intel/common/cache_as_ram.S: Add macro to find a free MTRR

This adds a macro to find an available MTRR(s) to set up CAR.
This added complexity is not required on bootpaths without bootguard
but with bootguard MTRR's have already been set up by the ACM so
we need to figure out at runtime which ones are available.

Change-Id: I7d5442c75464cfb2b3611c63a472c8ee521c014d
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37190
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/soc/intel/common/block/cpu/car/cache_as_ram.S

@@ -12,6 +12,35 @@
 .section .init, "ax", @progbits
 
 .code32
+
+/*
+ * macro: find_free_mtrr
+ * Clobbers: %eax, %ebx, %ecx, %edx.
+ * Returns:
+ * %ebx contains the number of freely available MTRR's.
+ *      It should be checked against 0.
+ * %ecx holds the MTRR_BASE of the free MTRR.
+ */
+.macro find_free_mtrr
+	/* Figure out how many MTRRs we have */
+	mov	$MTRR_CAP_MSR, %ecx
+	rdmsr
+	movzb	%al, %ebx		/* Number of variable MTRRs */
+
+	/* Find a free variable MTRR */
+	movl	$MTRR_PHYS_MASK(0), %ecx
+1:
+	rdmsr
+	test	$MTRR_PHYS_MASK_VALID, %eax
+	jz	2f
+	addl	$2, %ecx
+	dec	%ebx
+	jnz	1b
+2:
+	/* %ecx needs to hold the MTRR_BASE */
+	decl	%ecx
+.endm
+
 .global bootblock_pre_c_entry
 bootblock_pre_c_entry:
 
@@ -82,15 +111,18 @@ clear_var_mtrr:
 	post_code(0x24)
 
 #if ((CONFIG_DCACHE_RAM_SIZE & (CONFIG_DCACHE_RAM_SIZE - 1)) == 0)
+	find_free_mtrr
+	test	%ebx, %ebx
+	jz	.halt_forever
+
 	/* Configure CAR region as write-back (WB) */
-	mov	$MTRR_PHYS_BASE(0), %ecx
 	mov	$CONFIG_DCACHE_RAM_BASE, %eax
 	or	$MTRR_TYPE_WRBACK, %eax
 	xor	%edx,%edx
 	wrmsr
 
 	/* Configure the MTRR mask for the size region */
-	mov	$MTRR_PHYS_MASK(0), %ecx
+	inc	%ecx
 	mov	$CONFIG_DCACHE_RAM_SIZE, %eax	/* size mask */
 	dec	%eax
 	not	%eax
@@ -98,14 +130,17 @@ clear_var_mtrr:
 	movl	%esi, %edx	/* edx <- MTRR_PHYS_MASK_HIGH */
 	wrmsr
 #elif (CONFIG_DCACHE_RAM_SIZE == 768 * KiB) /* 768 KiB */
+	find_free_mtrr
+	test	%ebx, %ebx
+	jz	.halt_forever
+
 	/* Configure CAR region as write-back (WB) */
-	mov	$MTRR_PHYS_BASE(0), %ecx
 	mov	$CONFIG_DCACHE_RAM_BASE, %eax
 	or	$MTRR_TYPE_WRBACK, %eax
 	xor	%edx,%edx
 	wrmsr
 
-	mov	$MTRR_PHYS_MASK(0), %ecx
+	incl	%ecx
 	mov	$(512 * KiB), %eax	/* size mask */
 	dec	%eax
 	not	%eax
@@ -113,13 +148,16 @@ clear_var_mtrr:
 	movl	%esi, %edx	/* edx <- MTRR_PHYS_MASK_HIGH */
 	wrmsr
 
-	mov	$MTRR_PHYS_BASE(1), %ecx
+	find_free_mtrr
+	test	%ebx, %ebx
+	jz	.halt_forever
+1:
 	mov	$(CONFIG_DCACHE_RAM_BASE + 512 * KiB), %eax
 	or	$MTRR_TYPE_WRBACK, %eax
 	xor	%edx,%edx
 	wrmsr
 
-	mov	$MTRR_PHYS_MASK(1), %ecx
+	incl	%ecx
 	mov	$(256 * KiB), %eax	/* size mask */
 	dec	%eax
 	not	%eax