[PATCH 10/18] arm64: Support hugetlb vmemmap optimization

[James Houghton <jthoughton@google.com>]

Now that HVO is architecturally sound for arm64, enable support. Add a
new Kconfig for this support, as it comes with a regression, described
below.

Add a comment about BBML2_NOABORT where HVO is relying on it.

If a late-onlined CPU does not support HVO (HW AF and BBML2_NOABORT)
while all boot CPUs do support HVO, the late CPU won't be onlined after
this commit when a kernel is compiled with
HUGETLB_PAGE_OPTIMIZE_VMEMMAP=y.

Signed-off-by: James Houghton <jthoughton@google.com>
---
 arch/arm64/Kconfig   | 12 ++++++++++++
 mm/hugetlb_vmemmap.c |  4 ++++
 2 files changed, 16 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b3afe0688919..cf4bd6bd7f11 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -259,6 +259,7 @@ config ARM64
 	select USER_STACKTRACE_SUPPORT
 	select VDSO_GETRANDOM
 	select VMAP_STACK
+	select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP if ARM64_WANT_OPTIMIZE_HUGETLB_VMEMMAP
 	help
 	  ARM 64-bit (AArch64) Linux support.
 
@@ -1607,6 +1608,17 @@ config ARCH_SPARSEMEM_ENABLE
 	def_bool y
 	select SPARSEMEM_VMEMMAP_ENABLE
 
+config ARM64_WANT_OPTIMIZE_HUGETLB_VMEMMAP
+	bool "Enable support for HugeTLB Vmemmap Optimization (HVO)"
+	default n
+	depends on HUGETLB_PAGE && SPARSEMEM_VMEMMAP && ARM64_HW_AFDBM
+	help
+	  Enable support for HVO. When enabled, late-onlining of CPUs that do
+	  not support features required by HVO will not be onlined if all boot
+	  CPUs do have such support.
+
+	  If unsure, say N
+
 config HW_PERF_EVENTS
 	def_bool y
 	depends on ARM_PMU
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 21377f795293..977249e22ed0 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -86,6 +86,10 @@ static int vmemmap_split_pmd(pmd_t *pmd, struct page *head, unsigned long start,
 
 		/* Make pte visible before pmd. See comment in pmd_install(). */
 		smp_wmb();
+		/*
+		 * On arm64, this requires BBML2_NOABORT. Its support has
+		 * already been checked.
+		 */
 		pmd_populate_kernel(&init_mm, pmd, pgtable);
 		if (!(walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH))
 			flush_tlb_kernel_range(start, start + PMD_SIZE);
-- 
2.55.0.795.g602f6c329a-goog