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

yunhui cui
2024-07-02T09:48:26Z

Hi Alexandre,

On Tue, Jul 2, 2024 at 4:54 PM Alexandre Ghiti <alexghiti@rivosinc.com> wrote:
>
> 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)

We discussed in the previous version that when executing blt a0, zero,
_new_vmalloc_restore_context, there is no need to save a1, a2 first,
right?

> +
> + 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
>

Thanks，
Yunhui