Hi Alexandre, On Mon, Jun 3, 2024 at 8:02 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote: > > Hi Yunhui, > > On Mon, Jun 3, 2024 at 4:26 AM yunhui cui <cuiyunhui@xxxxxxxxxxxxx> wrote: > > > > 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? > > At first, I used percpu variables. But then I realized that percpu > areas are allocated in the vmalloc area, so if somehow we take a trap > when accessing the new_vmalloc percpu variable, we could not recover > from this as we would trap forever in new_vmalloc_check. But > admittedly, not sure that can happen. > > And how would that remove a0, a1 and a2 from thread_info? We'd still > need to save some registers somewhere to access the percpu variable > right? > > > 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? > > If we write that in C, we don't have the control over the allocated > registers and then we can't correctly save the context. If we use C language, new_vmalloc_check is written just like do_irq(), then we need _save_context, but for new_vmalloc_check, it is not worth the loss, because exceptions from user mode do not need new_vmalloc_check, which also shows that it is reasonable to put new_vmalloc_check after _restore_kernel_tpsp. Saving is necessary. We can save a0, a1, a2 without using thread_info. We can choose to save on the kernel stack of the current tp, but we need to add the following instructions: REG_S sp, TASK_TI_USER_SP(tp) REG_L sp, TASK_TI_KERNEL_SP(tp) addi sp, sp, -(PT_SIZE_ON_STACK) It seems that saving directly on thread_info is more direct, but saving on the kernel stack is more logically consistent, and there is no need to increase the size of thread_info. As for the current status of the patch, there are two points that can be optimized: 1. Some chip hardware implementations may not cache TLB invalid entries, so it doesn't matter whether svvptc is available or not. Can we consider adding a CONFIG_RISCV_SVVPTC to control it? 2. .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) When executing blt a0, zero, _new_vmalloc_restore_context, you can not save a1, a2 first > > Thanks for your interest in this patchset :) > > Alex > > > > > Thanks, > > Yunhui Thanks, Yunhui
