[PATCH v3 3/4] riscv: Stop emitting preventive sfence.vma for new vmalloc mappings

Alexandre Ghiti
2024-07-02T08:54:07Z

In 6.5, we removed the vmalloc fault path because that can't work (see
[1] [2]). Then in order to make sure that new page table entries were
seen by the page table walker, we had to preventively emit a sfence.vma
on all harts [3] but this solution is very costly since it relies on IPI.

And even there, we could end up in a loop of vmalloc faults if a vmalloc
allocation is done in the IPI path (for example if it is traced, see
[4]), which could result in a kernel stack overflow.

Those preventive sfence.vma needed to be emitted because:

- if the uarch caches invalid entries, the new mapping may not be
observed by the page table walker and an invalidation may be needed.
- if the uarch does not cache invalid entries, a reordered access
could "miss" the new mapping and traps: in that case, we would actually
only need to retry the access, no sfence.vma is required.

So this patch removes those preventive sfence.vma and actually handles
the possible (and unlikely) exceptions. And since the kernel stacks
mappings lie in the vmalloc area, this handling must be done very early
when the trap is taken, at the very beginning of handle_exception: this
also rules out the vmalloc allocations in the fault path.

Link: https://lore.kernel.org/linux-riscv/20230531093817.665799-1-bjorn@kernel.org/ [1]
Link: https://lore.kernel.org/linux-riscv/20230801090927.2018653-1-dylan@andestech.com [2]
Link: https://lore.kernel.org/linux-riscv/20230725132246.817726-1-alexghiti@rivosinc.com/ [3]
Link: https://lore.kernel.org/lkml/20200508144043.13893-1-joro@8bytes.org/ [4]
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
arch/riscv/include/asm/cacheflush.h | 18 +++++-
arch/riscv/include/asm/thread_info.h | 5 ++
arch/riscv/kernel/asm-offsets.c | 5 ++
arch/riscv/kernel/entry.S | 84 ++++++++++++++++++++++++++++
arch/riscv/mm/init.c | 2 +
5 files changed, 113 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h
index ce79c558a4c8..8de73f91bfa3 100644
--- a/arch/riscv/include/asm/cacheflush.h
+++ b/arch/riscv/include/asm/cacheflush.h
@@ -46,7 +46,23 @@ do { \
} while (0)

#ifdef CONFIG_64BIT
-#define flush_cache_vmap(start, end) flush_tlb_kernel_range(start, end)
+extern u64 new_vmalloc[NR_CPUS / sizeof(u64) + 1];
+extern char _end[];
+#define flush_cache_vmap flush_cache_vmap
+static inline void flush_cache_vmap(unsigned long start, unsigned long end)
+{
+ if (is_vmalloc_or_module_addr((void *)start)) {
+ int i;
+
+ /*
+ * We don't care if concurrently a cpu resets this value since
+ * the only place this can happen is in handle_exception() where
+ * an sfence.vma is emitted.
+ */
+ for (i = 0; i < ARRAY_SIZE(new_vmalloc); ++i)
+ new_vmalloc[i] = -1ULL;
+ }
+}
#define flush_cache_vmap_early(start, end) local_flush_tlb_kernel_range(start, end)
#endif

diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index 5d473343634b..32631acdcdd4 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -60,6 +60,11 @@ struct thread_info {
void *scs_base;
void *scs_sp;
#endif
+ /*
+ * Used in handle_exception() to save a0, a1 and a2 before knowing if we
+ * can access the kernel stack.
+ */
+ unsigned long a0, a1, a2;
};

#ifdef CONFIG_SHADOW_CALL_STACK
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index b09ca5f944f7..29c0734f2972 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -36,6 +36,8 @@ void asm_offsets(void)
OFFSET(TASK_THREAD_S9, task_struct, thread.s[9]);
OFFSET(TASK_THREAD_S10, task_struct, thread.s[10]);
OFFSET(TASK_THREAD_S11, task_struct, thread.s[11]);
+
+ OFFSET(TASK_TI_CPU, task_struct, thread_info.cpu);
OFFSET(TASK_TI_FLAGS, task_struct, thread_info.flags);
OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
@@ -43,6 +45,9 @@ void asm_offsets(void)
#ifdef CONFIG_SHADOW_CALL_STACK
OFFSET(TASK_TI_SCS_SP, task_struct, thread_info.scs_sp);
#endif
+ OFFSET(TASK_TI_A0, task_struct, thread_info.a0);
+ OFFSET(TASK_TI_A1, task_struct, thread_info.a1);
+ OFFSET(TASK_TI_A2, task_struct, thread_info.a2);

OFFSET(TASK_TI_CPU_NUM, task_struct, thread_info.cpu);
OFFSET(TASK_THREAD_F0, task_struct, thread.fstate.f[0]);
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 68a24cf9481a..822311266a12 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -19,6 +19,78 @@

.section .irqentry.text, "ax"

+.macro new_vmalloc_check
+ REG_S a0, TASK_TI_A0(tp)
+ REG_S a1, TASK_TI_A1(tp)
+ REG_S a2, TASK_TI_A2(tp)
+
+ csrr a0, CSR_CAUSE
+ /* Exclude IRQs */
+ blt a0, zero, _new_vmalloc_restore_context
+ /* Only check new_vmalloc if we are in page/protection fault */
+ li a1, EXC_LOAD_PAGE_FAULT
+ beq a0, a1, _new_vmalloc_kernel_address
+ li a1, EXC_STORE_PAGE_FAULT
+ beq a0, a1, _new_vmalloc_kernel_address
+ li a1, EXC_INST_PAGE_FAULT
+ bne a0, a1, _new_vmalloc_restore_context
+
+_new_vmalloc_kernel_address:
+ /* Is it a kernel address? */
+ csrr a0, CSR_TVAL
+ bge a0, zero, _new_vmalloc_restore_context
+
+ /* Check if a new vmalloc mapping appeared that could explain the trap */
+
+ /*
+ * Computes:
+ * a0 = &new_vmalloc[BIT_WORD(cpu)]
+ * a1 = BIT_MASK(cpu)
+ */
+ REG_L a2, TASK_TI_CPU(tp)
+ /*
+ * Compute the new_vmalloc element position:
+ * (cpu / 64) * 8 = (cpu >> 6) << 3
+ */
+ srli a1, a2, 6
+ slli a1, a1, 3
+ la a0, new_vmalloc
+ add a0, a0, a1
+ /*
+ * Compute the bit position in the new_vmalloc element:
+ * bit_pos = cpu % 64 = cpu - (cpu / 64) * 64 = cpu - (cpu >> 6) << 6
+ * = cpu - ((cpu >> 6) << 3) << 3
+ */
+ slli a1, a1, 3
+ sub a1, a2, a1
+ /* Compute the "get mask": 1 << bit_pos */
+ li a2, 1
+ sll a1, a2, a1
+
+ /* Check the value of new_vmalloc for this cpu */
+ REG_L a2, 0(a0)
+ and a2, a2, a1
+ beq a2, zero, _new_vmalloc_restore_context
+
+ /* Atomically reset the current cpu bit in new_vmalloc */
+ amoxor.w a0, a1, (a0)
+
+ /* Only emit a sfence.vma if the uarch caches invalid entries */
+ ALTERNATIVE("sfence.vma", "nop", 0, RISCV_ISA_EXT_SVVPTC, 1)
+
+ REG_L a0, TASK_TI_A0(tp)
+ REG_L a1, TASK_TI_A1(tp)
+ REG_L a2, TASK_TI_A2(tp)
+ csrw CSR_SCRATCH, x0
+ sret
+
+_new_vmalloc_restore_context:
+ REG_L a0, TASK_TI_A0(tp)
+ REG_L a1, TASK_TI_A1(tp)
+ REG_L a2, TASK_TI_A2(tp)
+.endm
+
+
SYM_CODE_START(handle_exception)
/*
* If coming from userspace, preserve the user thread pointer and load
@@ -30,6 +102,18 @@ SYM_CODE_START(handle_exception)

.Lrestore_kernel_tpsp:
csrr tp, CSR_SCRATCH
+
+ /*
+ * The RISC-V kernel does not eagerly emit a sfence.vma after each
+ * new vmalloc mapping, which may result in exceptions:
+ * - if the uarch caches invalid entries, the new mapping would not be
+ * observed by the page table walker and an invalidation is needed.
+ * - if the uarch does not cache invalid entries, a reordered access
+ * could "miss" the new mapping and traps: in that case, we only need
+ * to retry the access, no sfence.vma is required.
+ */
+ new_vmalloc_check
+
REG_S sp, TASK_TI_KERNEL_SP(tp)

#ifdef CONFIG_VMAP_STACK
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index e3405e4b99af..2367a156c33b 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -36,6 +36,8 @@

#include "../kernel/head.h"

+u64 new_vmalloc[NR_CPUS / sizeof(u64) + 1];
+
struct kernel_mapping kernel_map __ro_after_init;
EXPORT_SYMBOL(kernel_map);
#ifdef CONFIG_XIP_KERNEL
--
2.39.2