Hi Alexandre, On Thu, Feb 1, 2024 at 12:03 AM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> 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@xxxxxxxxxx/ [1] > Link: https://lore.kernel.org/linux-riscv/20230801090927.2018653-1-dylan@xxxxxxxxxxxxx [2] > Link: https://lore.kernel.org/linux-riscv/20230725132246.817726-1-alexghiti@xxxxxxxxxxxx/ [3] > Link: https://lore.kernel.org/lkml/20200508144043.13893-1-joro@xxxxxxxxxx/ [4] > Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> > --- > 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 a129dac4521d..b0d631701757 100644 > --- a/arch/riscv/include/asm/cacheflush.h > +++ b/arch/riscv/include/asm/cacheflush.h > @@ -37,7 +37,23 @@ static inline void flush_dcache_page(struct page *page) > flush_icache_mm(vma->vm_mm, 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 a03129f40c46..939ddc0e3c6e 100644 > --- a/arch/riscv/kernel/asm-offsets.c > +++ b/arch/riscv/kernel/asm-offsets.c > @@ -35,6 +35,8 @@ 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); > @@ -42,6 +44,9 @@ 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 9d1a305d5508..c1ffaeaba7aa 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 eafc4c2200f2..54c9fdeda11e 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 > > Can we consider using new_vmalloc as a percpu variable, so that we don't need to add a0/1/2 in thread_info? Also, try not to do too much calculation logic in new_vmalloc_check, after all, handle_exception is a high-frequency path. In this case, can we consider writing new_vmalloc_check in C language to increase readability? Thanks, Yunhui
