Re: [External] [PATCH RFC/RFT v2 3/4] riscv: Stop emitting preventive sfence.vma for new vmalloc mappings
From: Alexandre Ghiti
Date: Mon Jun 03 2024 - 08:02:35 EST
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.
Thanks for your interest in this patchset :)
Alex
>
> Thanks,
> Yunhui