linux-kernel - [PATCH RFC/RFT v2 4/4] riscv: Stop emitting preventive sfence.vma for new userspace mappings with Svvptc

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Date: Wed, 31 Jan 2024 16:59:29 +0100
From: Alexandre Ghiti <alexghiti@...osinc.com>
To: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ved Shanbhogue <ved@...osinc.com>,
	Matt Evans <mev@...osinc.com>,
	Dylan Jhong <dylan@...estech.com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-mips@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	linux-riscv@...ts.infradead.org,
	linux-mm@...ck.org
Cc: Alexandre Ghiti <alexghiti@...osinc.com>
Subject: [PATCH RFC/RFT v2 4/4] riscv: Stop emitting preventive sfence.vma for new userspace mappings with Svvptc

The preventive sfence.vma were emitted because new mappings must be made
visible to the page table walker but Svvptc guarantees that xRET act as
a fence, so no need to sfence.vma for the uarchs that implement this
extension.

This allows to drastically reduce the number of sfence.vma emitted:

* Ubuntu boot to login:
Before: ~630k sfence.vma
After:  ~200k sfence.vma

* ltp - mmapstress01
Before: ~45k
After:  ~6.3k

* lmbench - lat_pagefault
Before: ~665k
After:   832 (!)

* lmbench - lat_mmap
Before: ~546k
After:   718 (!)

Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
---
 arch/riscv/include/asm/pgtable.h | 16 +++++++++++++++-
 arch/riscv/mm/pgtable.c          | 13 +++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 0c94260b5d0c..50986e4c4601 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -473,6 +473,9 @@ static inline void update_mmu_cache_range(struct vm_fault *vmf,
 		struct vm_area_struct *vma, unsigned long address,
 		pte_t *ptep, unsigned int nr)
 {
+	asm_volatile_goto(ALTERNATIVE("nop", "j %l[svvptc]", 0, RISCV_ISA_EXT_SVVPTC, 1)
+			  : : : : svvptc);
+
 	/*
 	 * The kernel assumes that TLBs don't cache invalid entries, but
 	 * in RISC-V, SFENCE.VMA specifies an ordering constraint, not a
@@ -482,12 +485,23 @@ static inline void update_mmu_cache_range(struct vm_fault *vmf,
 	 */
 	while (nr--)
 		local_flush_tlb_page(address + nr * PAGE_SIZE);
+
+svvptc:
+	/*
+	 * Svvptc guarantees that xRET act as a fence, so when the uarch does
+	 * not cache invalid entries, we don't have to do anything.
+	 */
+	;
 }
 #define update_mmu_cache(vma, addr, ptep) \
 	update_mmu_cache_range(NULL, vma, addr, ptep, 1)
 
 #define __HAVE_ARCH_UPDATE_MMU_TLB
-#define update_mmu_tlb update_mmu_cache
+static inline void update_mmu_tlb(struct vm_area_struct *vma,
+				  unsigned long address, pte_t *ptep)
+{
+	flush_tlb_range(vma, address, address + PAGE_SIZE);
+}
 
 static inline void update_mmu_cache_pmd(struct vm_area_struct *vma,
 		unsigned long address, pmd_t *pmdp)
diff --git a/arch/riscv/mm/pgtable.c b/arch/riscv/mm/pgtable.c
index ef887efcb679..99ed389e4c8a 100644
--- a/arch/riscv/mm/pgtable.c
+++ b/arch/riscv/mm/pgtable.c
@@ -9,6 +9,9 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
 			  unsigned long address, pte_t *ptep,
 			  pte_t entry, int dirty)
 {
+	asm_volatile_goto(ALTERNATIVE("nop", "j %l[svvptc]", 0, RISCV_ISA_EXT_SVVPTC, 1)
+			  : : : : svvptc);
+
 	if (!pte_same(ptep_get(ptep), entry))
 		__set_pte_at(ptep, entry);
 	/*
@@ -16,6 +19,16 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
 	 * the case that the PTE changed and the spurious fault case.
 	 */
 	return true;
+
+svvptc:
+	if (!pte_same(ptep_get(ptep), entry)) {
+		__set_pte_at(ptep, entry);
+		/* Here only not svadu is impacted */
+		flush_tlb_page(vma, address);
+		return true;
+	}
+
+	return false;
 }
 
 int ptep_test_and_clear_young(struct vm_area_struct *vma,
-- 
2.39.2