[PATCH v3 5/7] x86/boot: Drop RIP_REL_REF() uses from early mapping code

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

Now that __startup_64() is built using -fPIC, RIP_REL_REF() has become a
NOP and can be removed. Only some occurrences of rip_rel_ptr() will
remain, to explicitly take the address of certain global structures in
the 1:1 mapping of memory.

While at it, update the code comment to describe why this is needed.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/startup/map_kernel.c | 41 ++++++++++----------
 1 file changed, 21 insertions(+), 20 deletions(-)

diff --git a/arch/x86/boot/startup/map_kernel.c b/arch/x86/boot/startup/map_kernel.c
index 5f1b7e0ba26e..0eac3f17dbd3 100644
--- a/arch/x86/boot/startup/map_kernel.c
+++ b/arch/x86/boot/startup/map_kernel.c
@@ -26,12 +26,12 @@ static inline bool check_la57_support(void)
 	if (!(native_read_cr4() & X86_CR4_LA57))
 		return false;
 
-	RIP_REL_REF(__pgtable_l5_enabled)	= 1;
-	RIP_REL_REF(pgdir_shift)		= 48;
-	RIP_REL_REF(ptrs_per_p4d)		= 512;
-	RIP_REL_REF(page_offset_base)		= __PAGE_OFFSET_BASE_L5;
-	RIP_REL_REF(vmalloc_base)		= __VMALLOC_BASE_L5;
-	RIP_REL_REF(vmemmap_base)		= __VMEMMAP_BASE_L5;
+	__pgtable_l5_enabled	= 1;
+	pgdir_shift		= 48;
+	ptrs_per_p4d		= 512;
+	page_offset_base	= __PAGE_OFFSET_BASE_L5;
+	vmalloc_base		= __VMALLOC_BASE_L5;
+	vmemmap_base		= __VMEMMAP_BASE_L5;
 
 	return true;
 }
@@ -81,12 +81,14 @@ static unsigned long __head sme_postprocess_startup(struct boot_params *bp,
 	return sme_get_me_mask();
 }
 
-/* Code in __startup_64() can be relocated during execution, but the compiler
- * doesn't have to generate PC-relative relocations when accessing globals from
- * that function. Clang actually does not generate them, which leads to
- * boot-time crashes. To work around this problem, every global pointer must
- * be accessed using RIP_REL_REF(). Kernel virtual addresses can be determined
- * by subtracting p2v_offset from the RIP-relative address.
+/*
+ * This code is compiled using PIC codegen because it will execute from the
+ * early 1:1 mapping of memory, which deviates from the mapping expected by the
+ * linker. Due to this deviation, taking the address of a global variable will
+ * produce an ambiguous result when using the plain & operator.  Instead,
+ * rip_rel_ptr() must be used, which will return the RIP-relative address in
+ * the 1:1 mapping of memory. Kernel virtual addresses can be determined by
+ * subtracting p2v_offset from the RIP-relative address.
  */
 unsigned long __head __startup_64(unsigned long p2v_offset,
 				  struct boot_params *bp)
@@ -113,8 +115,7 @@ unsigned long __head __startup_64(unsigned long p2v_offset,
 	 * Compute the delta between the address I am compiled to run at
 	 * and the address I am actually running at.
 	 */
-	load_delta = __START_KERNEL_map + p2v_offset;
-	RIP_REL_REF(phys_base) = load_delta;
+	phys_base = load_delta = __START_KERNEL_map + p2v_offset;
 
 	/* Is the address not 2M aligned? */
 	if (load_delta & ~PMD_MASK)
@@ -138,11 +139,11 @@ unsigned long __head __startup_64(unsigned long p2v_offset,
 		pgd[pgd_index(__START_KERNEL_map)] = (pgdval_t)p4d | _PAGE_TABLE;
 	}
 
-	RIP_REL_REF(level3_kernel_pgt)[PTRS_PER_PUD - 2].pud += load_delta;
-	RIP_REL_REF(level3_kernel_pgt)[PTRS_PER_PUD - 1].pud += load_delta;
+	level3_kernel_pgt[PTRS_PER_PUD - 2].pud += load_delta;
+	level3_kernel_pgt[PTRS_PER_PUD - 1].pud += load_delta;
 
 	for (i = FIXMAP_PMD_TOP; i > FIXMAP_PMD_TOP - FIXMAP_PMD_NUM; i--)
-		RIP_REL_REF(level2_fixmap_pgt)[i].pmd += load_delta;
+		level2_fixmap_pgt[i].pmd += load_delta;
 
 	/*
 	 * Set up the identity mapping for the switchover.  These
@@ -153,12 +154,12 @@ unsigned long __head __startup_64(unsigned long p2v_offset,
 
 	pud = &early_pgts[0]->pmd;
 	pmd = &early_pgts[1]->pmd;
-	RIP_REL_REF(next_early_pgt) = 2;
+	next_early_pgt = 2;
 
 	pgtable_flags = _KERNPG_TABLE_NOENC + sme_get_me_mask();
 
 	if (la57) {
-		p4d = &early_pgts[RIP_REL_REF(next_early_pgt)++]->pmd;
+		p4d = &early_pgts[next_early_pgt++]->pmd;
 
 		i = (physaddr >> PGDIR_SHIFT) % PTRS_PER_PGD;
 		pgd[i + 0] = (pgdval_t)p4d + pgtable_flags;
@@ -179,7 +180,7 @@ unsigned long __head __startup_64(unsigned long p2v_offset,
 
 	pmd_entry = __PAGE_KERNEL_LARGE_EXEC & ~_PAGE_GLOBAL;
 	/* Filter out unsupported __PAGE_KERNEL_* bits: */
-	pmd_entry &= RIP_REL_REF(__supported_pte_mask);
+	pmd_entry &= __supported_pte_mask;
 	pmd_entry += sme_get_me_mask();
 	pmd_entry +=  physaddr;
 
-- 
2.49.0.504.g3bcea36a83-goog