sec/intel/txt: Split MTRR setup ASM code into a macro

If necessary, SCLEAN needs to run in early romstage, where DRAM is not
working yet. In fact, that the DRAM isn't working is the reason to run
SCLEAN in the first place. Before running GETSEC, CAR needs to be torn
down, as MTRRs have to be reprogrammed to cache the BIOS ACM. Further,
running SCLEAN leaves the system in an undefined state, where the only
sane thing to do is reset the platform. Thus, invoking SCLEAN requires
specific assembly prologue and epilogue sections before and after MTRR
setup, and neither DRAM nor CAR may be relied upon for the MTRR setup.

In order to handle this without duplicating the MTRR setup code, place
it in a macro on a separate file. This needs to be a macro because the
call and return instructions rely on the stack being usable, and it is
not the case for SCLEAN. The MTRR code clobbers many registers, but no
other choice remains when the registers cannot be saved anywhere else.

Tested on Asrock B85M Pro4, BIOS ACM can still be launched.

Change-Id: I2f5e82f57b458ca1637790ddc1ddc14bba68ac49
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46603
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

2 files changed, 84 insertions, 64 deletions

diff --git a/src/security/intel/txt/getsec_enteraccs.S b/src/security/intel/txt/getsec_enteraccs.S
index be038b0607..be3a1b4f0b 100644
--- a/src/security/intel/txt/getsec_enteraccs.S
+++ b/src/security/intel/txt/getsec_enteraccs.S
@@ -4,7 +4,7 @@
 #include <cpu/x86/cr.h>
 #include <cpu/x86/msr.h>
 
-#define MTRR_HIGH_MASK $((1 << (CONFIG_CPU_ADDR_BITS - 32)) - 1)
+#include "getsec_mtrr_setup.inc"
 
 .macro PUSH_MSR x
 	movl	$(\x), %ecx
@@ -166,15 +166,6 @@ cond_clear_var_mtrrs:
 	 * Intel TXT Software Development Guide (Document: 315168-015)
 	 */
 
-	/*
-	 * Important note: The MTRRs must cache less than a page (4 KiB)
-	 * of unused memory after the BIOS ACM. Failure to do so will
-	 * result in a TXT reset with Class Code 5, Major Error Code 2.
-	 *
-	 * The caller must have checked that there are enough variable
-	 * MTRRs to cache the ACM size prior to invoking this routine.
-	 */
-
 	/* Determine size of AC module */
 	movl	12(%ebp), %eax			/* %eax = acmbase */
 	movl	$1, %ebx
@@ -191,60 +182,15 @@ cond_clear_var_mtrrs:
 	movd	%eax, %xmm0			/* XMM0: Base address of next MTRR */
 	movd	%ebx, %xmm1			/* XMM1: Remaining size to cache */
 
-	/* Get the number of variable MTRRs */
-	movl	$(MTRR_CAP_MSR), %ecx
-	rdmsr
-	andl	$(0xff), %eax
-
-	/* Initialize ECX */
-	movl	$(MTRR_PHYS_BASE(0)), %ecx
-
-	jmp	cond_allocate_var_mtrrs
-
-body_allocate_var_mtrrs:
-
-	/* Program MTRR base */
-	xorl	%edx, %edx
-	movd	%xmm0, %eax
-	orl	$(MTRR_TYPE_WRBACK), %eax
-	wrmsr
-	incl	%ecx				/* Move index to MTRR_PHYS_MASK */
-
-	/* Temporarily transfer MSR index to EDX so that CL can be used */
-	movl	%ecx, %edx
-
-	/* Determine next size to cache */
-	bsr	%ebx, %ecx
-	movl	$(1), %ebx
-	shl	%cl, %ebx			/* Can only use CL here */
-
-	/* Restore ECX */
-	movl	%edx, %ecx
-
-	/* Update saved base address */
-	addl	%ebx, %eax
-	movd	%eax, %xmm0
-
-	/* Update saved remaining size */
-	movd	%xmm1, %eax
-	subl	%ebx, %eax
-	movd	%eax, %xmm1
-
-	/* Program MTRR mask */
-	movl	MTRR_HIGH_MASK, %edx
-	xorl	%eax, %eax
-	subl	%ebx, %eax			/* %eax = 4GIB - size to cache */
-	orl	$(MTRR_PHYS_MASK_VALID), %eax
-	wrmsr
-	incl	%ecx				/* Move index to next MTRR_PHYS_BASE */
-
-cond_allocate_var_mtrrs:
-
-	/* Check if we still need to cache something */
-	movd	%xmm1, %ebx
-	andl	%ebx, %ebx
-
-	jnz	body_allocate_var_mtrrs
+	/*
+	 * Important note: The MTRRs must cache less than a page (4 KiB)
+	 * of unused memory after the BIOS ACM. Failure to do so will
+	 * result in a TXT reset with Class Code 5, Major Error Code 2.
+	 *
+	 * The caller must have checked that there are enough variable
+	 * MTRRs to cache the ACM size prior to invoking this routine.
+	 */
+	SET_UP_MTRRS_FOR_BIOS_ACM
 
 	/*
 	 * Now that the variable MTRRs have been set up, enable them.
diff --git a/src/security/intel/txt/getsec_mtrr_setup.inc b/src/security/intel/txt/getsec_mtrr_setup.inc
new file mode 100644
index 0000000000..15e8cc17ad
--- /dev/null
+++ b/src/security/intel/txt/getsec_mtrr_setup.inc
@@ -0,0 +1,74 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <cpu/x86/mtrr.h>
+#include <cpu/x86/msr.h>
+
+#define MTRR_HIGH_MASK $((1 << (CONFIG_CPU_ADDR_BITS - 32)) - 1)
+
+/*
+ * Configure the MTRRs to cache the BIOS ACM. No general-purpose
+ * registers are preserved. Inputs are taken from SSE registers:
+ *
+ *    %xmm0: BIOS ACM base
+ *    %xmm1: BIOS ACM size
+ *
+ * These two SSE registers are not preserved, but the others are.
+ */
+.macro SET_UP_MTRRS_FOR_BIOS_ACM
+
+	/* Get the number of variable MTRRs */
+	movl	$(MTRR_CAP_MSR), %ecx
+	rdmsr
+	andl	$(0xff), %eax
+
+	/* Initialize ECX */
+	movl	$(MTRR_PHYS_BASE(0)), %ecx
+
+	jmp	cond_allocate_var_mtrrs
+
+body_allocate_var_mtrrs:
+
+	/* Program MTRR base */
+	xorl	%edx, %edx
+	movd	%xmm0, %eax
+	orl	$(MTRR_TYPE_WRBACK), %eax
+	wrmsr
+	incl	%ecx				/* Move index to MTRR_PHYS_MASK */
+
+	/* Temporarily transfer MSR index to EDX so that CL can be used */
+	movl	%ecx, %edx
+
+	/* Determine next size to cache */
+	bsr	%ebx, %ecx
+	movl	$(1), %ebx
+	shl	%cl, %ebx			/* Can only use CL here */
+
+	/* Restore ECX */
+	movl	%edx, %ecx
+
+	/* Update saved base address */
+	addl	%ebx, %eax
+	movd	%eax, %xmm0
+
+	/* Update saved remaining size */
+	movd	%xmm1, %eax
+	subl	%ebx, %eax
+	movd	%eax, %xmm1
+
+	/* Program MTRR mask */
+	movl	MTRR_HIGH_MASK, %edx
+	xorl	%eax, %eax
+	subl	%ebx, %eax			/* %eax = 4GIB - size to cache */
+	orl	$(MTRR_PHYS_MASK_VALID), %eax
+	wrmsr
+	incl	%ecx				/* Move index to next MTRR_PHYS_BASE */
+
+cond_allocate_var_mtrrs:
+
+	/* Check if we still need to cache something */
+	movd	%xmm1, %ebx
+	andl	%ebx, %ebx
+
+	jnz	body_allocate_var_mtrrs
+
+.endm