diff options
author | Palmer Dabbelt <palmer@rivosinc.com> | 2024-09-15 20:16:12 -0700 |
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committer | Palmer Dabbelt <palmer@rivosinc.com> | 2024-09-15 20:58:24 -0700 |
commit | 7e340f4fad46b766705be96f5d1c764a397a7a36 (patch) | |
tree | 04800ea59f316eb806ebc3e24d05abf9d1233b7c /arch/riscv/kernel/entry.S | |
parent | 1845d381f28063a3b68e9e148d5a7f01d6be8721 (diff) | |
parent | 7a21b2e370dab780ddb3aa80f2a4c8ff97bddccc (diff) |
Merge patch series "Svvptc extension to remove preventive sfence.vma"
Alexandre Ghiti <alexghiti@rivosinc.com> says:
In RISC-V, after a new mapping is established, a sfence.vma needs to be
emitted for different reasons:
- if the uarch caches invalid entries, we need to invalidate it otherwise
we would trap on this invalid entry,
- if the uarch does not cache invalid entries, a reordered access could fail
to see the new mapping and then trap (sfence.vma acts as a fence).
We can actually avoid emitting those (mostly) useless and costly sfence.vma
by handling the traps instead:
- for new kernel mappings: only vmalloc mappings need to be taken care of,
other new mapping are rare and already emit the required sfence.vma if
needed.
That must be achieved very early in the exception path as explained in
patch 3, and this also fixes our fragile way of dealing with vmalloc faults.
- for new user mappings: Svvptc makes update_mmu_cache() a no-op but we can
take some gratuitous page faults (which are very unlikely though).
Patch 1 and 2 introduce Svvptc extension probing.
On our uarch that does not cache invalid entries and a 6.5 kernel, the
gains are measurable:
* Kernel boot: 6%
* ltp - mmapstress01: 8%
* lmbench - lat_pagefault: 20%
* lmbench - lat_mmap: 5%
Here are the corresponding numbers of sfence.vma emitted:
* Ubuntu boot to login:
Before: ~630k sfence.vma
After: ~200k sfence.vma
* ltp - mmapstress01
Before: ~45k
After: ~6.3k
* lmbench - lat_pagefault
Before: ~665k
After: 832 (!)
* lmbench - lat_mmap
Before: ~546k
After: 718 (!)
Thanks to Ved and Matt Evans for triggering the discussion that led to
this patchset!
* b4-shazam-merge:
riscv: Stop emitting preventive sfence.vma for new userspace mappings with Svvptc
riscv: Stop emitting preventive sfence.vma for new vmalloc mappings
dt-bindings: riscv: Add Svvptc ISA extension description
riscv: Add ISA extension parsing for Svvptc
Link: https://lore.kernel.org/r/20240717060125.139416-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Diffstat (limited to 'arch/riscv/kernel/entry.S')
-rw-r--r-- | arch/riscv/kernel/entry.S | 87 |
1 files changed, 87 insertions, 0 deletions
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S index ac2e908d4418..fc41472d512e 100644 --- a/arch/riscv/kernel/entry.S +++ b/arch/riscv/kernel/entry.S @@ -19,6 +19,79 @@ .section .irqentry.text, "ax" +.macro new_vmalloc_check + REG_S a0, TASK_TI_A0(tp) + csrr a0, CSR_CAUSE + /* Exclude IRQs */ + blt a0, zero, _new_vmalloc_restore_context_a0 + + REG_S a1, TASK_TI_A1(tp) + /* 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_a1 + +_new_vmalloc_kernel_address: + /* Is it a kernel address? */ + csrr a0, CSR_TVAL + bge a0, zero, _new_vmalloc_restore_context_a1 + + /* Check if a new vmalloc mapping appeared that could explain the trap */ + REG_S a2, TASK_TI_A2(tp) + /* + * 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.d 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 a2, TASK_TI_A2(tp) +_new_vmalloc_restore_context_a1: + REG_L a1, TASK_TI_A1(tp) +_new_vmalloc_restore_context_a0: + REG_L a0, TASK_TI_A0(tp) +.endm + + SYM_CODE_START(handle_exception) /* * If coming from userspace, preserve the user thread pointer and load @@ -30,6 +103,20 @@ SYM_CODE_START(handle_exception) .Lrestore_kernel_tpsp: csrr tp, CSR_SCRATCH + +#ifdef CONFIG_64BIT + /* + * 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 +#endif + REG_S sp, TASK_TI_KERNEL_SP(tp) #ifdef CONFIG_VMAP_STACK |