linux-kernel - [PATCH v2 10/10] riscv: Allow user to downgrade to sv39 when hw supports sv48

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Message-Id: <20210929145113.1935778-11-alexandre.ghiti@canonical.com>
Date:   Wed, 29 Sep 2021 16:51:13 +0200
From:   Alexandre Ghiti <alexandre.ghiti@...onical.com>
To:     Jonathan Corbet <corbet@....net>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Zong Li <zong.li@...ive.com>, Anup Patel <anup@...infault.org>,
        Atish Patra <Atish.Patra@....com>,
        Christoph Hellwig <hch@....de>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Heinrich Schuchardt <heinrich.schuchardt@...onical.com>,
        Mayuresh Chitale <mchitale@...tanamicro.com>,
        linux-doc@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linux-efi@...r.kernel.org, linux-arch@...r.kernel.org
Cc:     Alexandre Ghiti <alexandre.ghiti@...onical.com>
Subject: [PATCH v2 10/10] riscv: Allow user to downgrade to sv39 when hw supports sv48

This is made possible by using the mmu-type property of the cpu node of
the device tree.

By default, the kernel will boot with 4-level page table if the hw supports
it but it can be interesting for the user to select 3-level page table as
it is less memory consuming and faster since it requires less memory
accesses in case of a TLB miss.

Signed-off-by: Alexandre Ghiti <alexandre.ghiti@...onical.com>
---
 arch/riscv/mm/init.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index a304f2b3c178..676635f5d98a 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -641,10 +641,31 @@ static void __init disable_pgtable_l4(void)
  * then read SATP to see if the configuration was taken into account
  * meaning sv48 is supported.
  */
-static __init void set_satp_mode(void)
+static __init void set_satp_mode(uintptr_t dtb_pa)
 {
 	u64 identity_satp, hw_satp;
 	uintptr_t set_satp_mode_pmd;
+	int cpus_node;
+
+	/* Check if the user asked for sv39 explicitly in the device tree */
+	cpus_node = fdt_path_offset((void *)dtb_pa, "/cpus");
+	if (cpus_node >= 0) {
+		int node;
+
+		fdt_for_each_subnode(node, (void *)dtb_pa, cpus_node) {
+			const char *mmu_type = fdt_getprop((void *)dtb_pa, node,
+					"mmu-type", NULL);
+			if (!mmu_type)
+				continue;
+
+			if (!strcmp(mmu_type, "riscv,sv39")) {
+				disable_pgtable_l4();
+				return;
+			}
+
+			break;
+		}
+	}
 
 	set_satp_mode_pmd = ((unsigned long)set_satp_mode) & PMD_MASK;
 	create_pgd_mapping(early_pg_dir,
@@ -802,7 +823,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 #endif
 
 #if defined(CONFIG_64BIT) && !defined(CONFIG_XIP_KERNEL)
-	set_satp_mode();
+	set_satp_mode(dtb_pa);
 #endif
 
 	kernel_map.va_pa_offset = PAGE_OFFSET - kernel_map.phys_addr;
-- 
2.30.2