[PATCH v3 1/2] arm64: kpti-ng: simplify page table traversal logic

[Ard Biesheuvel]

Simplify the KPTI G-to-nG asm helper code by:
- pulling the 'table bit' test into the get/put macros so we can combine
  them and incorporate the entire loop;
- moving the 'table bit' test after the update of bit #11 so we no
  longer need separate next_xxx and skip_xxx labels;
- redefining the pmd/pud register aliases and the next_pmd/next_pud
  labels instead of branching to them if the number of configured page
  table levels is less than 3 or 4, respectively;
- folding the descriptor pointer increment into the LDR instructions.

No functional change intended, except for the fact that we now descend
into a next level table after setting bit #11 on its descriptor but this
should make no difference in practice.

While at it, switch to .L prefixed local labels so they don't clutter up
the symbol tables, kallsyms, etc, and clean up the indentation for
legibility.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/mm/proc.S | 97 +++++++-------------
 1 file changed, 34 insertions(+), 63 deletions(-)

diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index 50bbed947bec..5619c00f8cd4 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -202,19 +202,24 @@ SYM_FUNC_END(idmap_cpu_replace_ttbr1)
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
 	.pushsection ".idmap.text", "awx"
 
-	.macro	__idmap_kpti_get_pgtable_ent, type
+	.macro	kpti_mk_tbl_ng, type, num_entries
+	add	end_\type\()p, cur_\type\()p, #\num_entries * 8
+.Ldo_\type:
 	dc	cvac, cur_\()\type\()p		// Ensure any existing dirty
 	dmb	sy				// lines are written back before
-	ldr	\type, [cur_\()\type\()p]	// loading the entry
-	tbz	\type, #0, skip_\()\type	// Skip invalid and
-	tbnz	\type, #11, skip_\()\type	// non-global entries
-	.endm
-
-	.macro __idmap_kpti_put_pgtable_ent_ng, type
+	ldr	\type, [cur_\type\()p], #8	// loading the entry
+	tbz	\type, #0, .Lnext_\type		// Skip invalid and
+	tbnz	\type, #11, .Lnext_\type	// non-global entries
 	orr	\type, \type, #PTE_NG		// Same bit for blocks and pages
-	str	\type, [cur_\()\type\()p]	// Update the entry and ensure
+	str	\type, [cur_\type\()p, #-8]	// Update the entry and ensure
 	dmb	sy				// that it is visible to all
 	dc	civac, cur_\()\type\()p		// CPUs.
+	.ifnc	\type, pte
+	tbnz	\type, #1, .Lderef_\type
+	.endif
+.Lnext_\type:
+	cmp	cur_\type\()p, end_\type\()p
+	b.ne	.Ldo_\type
 	.endm
 
 /*
@@ -235,10 +240,8 @@ SYM_FUNC_START(idmap_kpti_install_ng_mappings)
 	pgd		.req	x7
 	cur_pudp	.req	x8
 	end_pudp	.req	x9
-	pud		.req	x10
 	cur_pmdp	.req	x11
 	end_pmdp	.req	x12
-	pmd		.req	x13
 	cur_ptep	.req	x14
 	end_ptep	.req	x15
 	pte		.req	x16
@@ -265,16 +268,8 @@ SYM_FUNC_START(idmap_kpti_install_ng_mappings)
 
 	/* Everybody is enjoying the idmap, so we can rewrite swapper. */
 	/* PGD */
-	mov	cur_pgdp, swapper_pa
-	add	end_pgdp, cur_pgdp, #(PTRS_PER_PGD * 8)
-do_pgd:	__idmap_kpti_get_pgtable_ent	pgd
-	tbnz	pgd, #1, walk_puds
-next_pgd:
-	__idmap_kpti_put_pgtable_ent_ng	pgd
-skip_pgd:
-	add	cur_pgdp, cur_pgdp, #8
-	cmp	cur_pgdp, end_pgdp
-	b.ne	do_pgd
+	mov		cur_pgdp, swapper_pa
+	kpti_mk_tbl_ng	pgd, PTRS_PER_PGD
 
 	/* Publish the updated tables and nuke all the TLBs */
 	dsb	sy
@@ -291,59 +286,35 @@ skip_pgd:
 	str	wzr, [flag_ptr]
 	ret
 
+.Lderef_pgd:
 	/* PUD */
-walk_puds:
-	.if CONFIG_PGTABLE_LEVELS > 3
+	.if		CONFIG_PGTABLE_LEVELS > 3
+	pud		.req	x10
 	pte_to_phys	cur_pudp, pgd
-	add	end_pudp, cur_pudp, #(PTRS_PER_PUD * 8)
-do_pud:	__idmap_kpti_get_pgtable_ent	pud
-	tbnz	pud, #1, walk_pmds
-next_pud:
-	__idmap_kpti_put_pgtable_ent_ng	pud
-skip_pud:
-	add	cur_pudp, cur_pudp, 8
-	cmp	cur_pudp, end_pudp
-	b.ne	do_pud
-	b	next_pgd
-	.else /* CONFIG_PGTABLE_LEVELS <= 3 */
-	mov	pud, pgd
-	b	walk_pmds
-next_pud:
-	b	next_pgd
+	kpti_mk_tbl_ng	pud, PTRS_PER_PUD
+	b		.Lnext_pgd
+	.else		/* CONFIG_PGTABLE_LEVELS <= 3 */
+	pud		.req	pgd
+	.set		.Lnext_pud, .Lnext_pgd
 	.endif
 
+.Lderef_pud:
 	/* PMD */
-walk_pmds:
-	.if CONFIG_PGTABLE_LEVELS > 2
+	.if		CONFIG_PGTABLE_LEVELS > 2
+	pmd		.req	x13
 	pte_to_phys	cur_pmdp, pud
-	add	end_pmdp, cur_pmdp, #(PTRS_PER_PMD * 8)
-do_pmd:	__idmap_kpti_get_pgtable_ent	pmd
-	tbnz	pmd, #1, walk_ptes
-next_pmd:
-	__idmap_kpti_put_pgtable_ent_ng	pmd
-skip_pmd:
-	add	cur_pmdp, cur_pmdp, #8
-	cmp	cur_pmdp, end_pmdp
-	b.ne	do_pmd
-	b	next_pud
-	.else /* CONFIG_PGTABLE_LEVELS <= 2 */
-	mov	pmd, pud
-	b	walk_ptes
-next_pmd:
-	b	next_pud
+	kpti_mk_tbl_ng	pmd, PTRS_PER_PMD
+	b		.Lnext_pud
+	.else		/* CONFIG_PGTABLE_LEVELS <= 2 */
+	pmd		.req	pgd
+	.set		.Lnext_pmd, .Lnext_pgd
 	.endif
 
+.Lderef_pmd:
 	/* PTE */
-walk_ptes:
 	pte_to_phys	cur_ptep, pmd
-	add	end_ptep, cur_ptep, #(PTRS_PER_PTE * 8)
-do_pte:	__idmap_kpti_get_pgtable_ent	pte
-	__idmap_kpti_put_pgtable_ent_ng	pte
-skip_pte:
-	add	cur_ptep, cur_ptep, #8
-	cmp	cur_ptep, end_ptep
-	b.ne	do_pte
-	b	next_pmd
+	kpti_mk_tbl_ng	pte, PTRS_PER_PTE
+	b		.Lnext_pmd
 
 	.unreq	cpu
 	.unreq	num_cpus
-- 
2.30.2