From: jinrui <[email protected]> When the cross-compiler defaults to an -march that includes the V (vector) extension, -O2 auto-vectorization generates vector instructions (e.g. vsetvli, vadd.vv) in Guest binary code. If the Guest executes a vector instruction while sstatus.VS is Off, an EXC_INST_ILLEGAL (scause=2) is raised. KVM's hedeleg delegates this exception to the Guest, but the selftest has no way to handle it as a bare-metal program, causing all Guest tests to fail.
In contrast, a real OS kernel handles this via riscv_v_first_use_handler(), which detects the vector instruction, sets sstatus.VS to Initial, and srets to re-execute. Fix this with four changes in processor.c: 1. Delete the now-unused guest_unexp_trap() handler, which is replaced by the full exception vector table. 2. In vm_arch_vcpu_add(), notify KVM that the Guest is allowed to use the V extension via __vcpu_set_reg(V, 1) (best-effort, silently ignores errors on hardware without V). Also replace the raw stvec handler with the full exception vector table, which provides save_context/restore_context for safe lazy enablement. 3. In route_exception(), add a lazy V enablement check that runs before any test-registered handler. When the cause is a non-IRQ EXC_INST_ILLEGAL and sstatus.VS is Off, set VS to Initial and return so the faulting instruction is re-executed via sret. Track the epc per-vCPU via a flexible array member to avoid infinite loops on hardware without V and eliminate races between concurrent vCPUs. 4. Make vm_init_vector_tables() idempotent by checking vm->handlers before allocation, so tests that manually call it (ebreak_test, arch_timer, sbi_pmu_test) do not leak memory. The check runs before test-registered EXC_INST_ILLEGAL handlers (e.g. sbi_pmu_test) to ensure V enablement takes priority. Tested on a riscv64 host with KVM enabled: all 20 KVM selftest binaries pass (arch_timer, ebreak_test, steal_time, get-reg-list, sbi_pmu_test, etc.). Signed-off-by: jinrui <[email protected]> --- Changes in v8: - Change 'ec' in route_exception() from int to unsigned long to match regs->cause (unsigned long), preventing signed truncation from bypassing the ec >= NR_EXCEPTIONS bounds check. Changes in v7: - Use unsigned int for vcpu_id, v_epc_capacity, and max_vcpus to prevent negative-value out-of-bounds array access on v_epc[]. Changes in v6: - Move struct handlers definition to the top of the file, before its first use in vm_arch_vcpu_add(). Changes in v5: - Allocate the per-vCPU epc table dynamically via a flexible array member sized by KVM_CAP_MAX_VCPUS rather than a hardcoded constant, making the concurrency fix safe for any vCPU count. Changes in v4: - Add vcpu_dump() in assert_on_unhandled_exception() to restore the register dump lost when guest_unexp_trap() was removed. Changes in v3: - Move v_available from a host-side static variable into struct handlers (Guest memory), fixing host-to-Guest synchronization. Changes in v2: - Add a v_available flag to prevent infinite exception loops on hosts without the V extension. - Use regs->status instead of reading the live sstatus CSR. --- .../selftests/kvm/lib/riscv/processor.c | 111 +++++++++++++++--- 1 file changed, 95 insertions(+), 16 deletions(-) diff --git a/tools/testing/selftests/kvm/lib/riscv/processor.c b/tools/testing/selftests/kvm/lib/riscv/processor.c index ded5429f3448..624f20f0f476 100644 --- a/tools/testing/selftests/kvm/lib/riscv/processor.c +++ b/tools/testing/selftests/kvm/lib/riscv/processor.c @@ -17,6 +17,13 @@ static gva_t exception_handlers; +struct handlers { + exception_handler_fn exception_handlers[NR_VECTORS][NR_EXCEPTIONS]; + bool v_available; + unsigned int v_epc_capacity; + unsigned long v_epc[]; +}; + bool __vcpu_has_ext(struct kvm_vcpu *vcpu, u64 ext) { unsigned long value = 0; @@ -297,14 +304,6 @@ void vcpu_arch_dump(FILE *stream, struct kvm_vcpu *vcpu, u8 indent) " T3: 0x%016lx T4: 0x%016lx T5: 0x%016lx T6: 0x%016lx\n", core.regs.t3, core.regs.t4, core.regs.t5, core.regs.t6); } - -static void __aligned(16) guest_unexp_trap(void) -{ - sbi_ecall(KVM_RISCV_SELFTESTS_SBI_EXT, - KVM_RISCV_SELFTESTS_SBI_UNEXP, - 0, 0, 0, 0, 0, 0); -} - void vcpu_arch_set_entry_point(struct kvm_vcpu *vcpu, void *guest_code) { vcpu_set_reg(vcpu, RISCV_CORE_REG(regs.pc), (unsigned long)guest_code); @@ -348,8 +347,33 @@ struct kvm_vcpu *vm_arch_vcpu_add(struct kvm_vm *vm, u32 vcpu_id) /* Setup sscratch for guest_get_vcpuid() */ vcpu_set_reg(vcpu, RISCV_GENERAL_CSR_REG(sscratch), vcpu_id); - /* Setup default exception vector of guest */ - vcpu_set_reg(vcpu, RISCV_GENERAL_CSR_REG(stvec), (unsigned long)guest_unexp_trap); + /* + * Enable the V (vector) extension in KVM so that the compiler can + * safely generate vector instructions (e.g. via -O2 auto- + * vectorization). Silently ignore errors; the test will still work + * without V. + */ + __vcpu_set_reg(vcpu, RISCV_ISA_EXT_REG(KVM_RISCV_ISA_EXT_V), 1); + + /* + * Use the full exception vector table (which provides lazy V + * extension enablement for EXC_INST_ILLEGAL in route_exception) + * as the default exception handler. vm_init_vector_tables() is + * idempotent; tests that call it again will get a no-op. + */ + vm_init_vector_tables(vm); + vcpu_init_vector_tables(vcpu); + + /* + * Record V extension availability in the handlers struct so that + * route_exception() (called from Guest context) can check it + * without relying on a host-side global variable. + */ + { + struct handlers *h = addr_gva2hva(vm, vm->handlers); + + h->v_available = __vcpu_has_isa_ext(vcpu, KVM_RISCV_ISA_EXT_V); + } return vcpu; } @@ -408,19 +432,17 @@ void assert_on_unhandled_exception(struct kvm_vcpu *vcpu) struct ucall uc; if (get_ucall(vcpu, &uc) == UCALL_UNHANDLED) { + vcpu_dump(stderr, vcpu, 2); TEST_FAIL("Unexpected exception (vector:0x%lx, ec:0x%lx)", uc.args[0], uc.args[1]); } } -struct handlers { - exception_handler_fn exception_handlers[NR_VECTORS][NR_EXCEPTIONS]; -}; - void route_exception(struct pt_regs *regs) { struct handlers *handlers = (struct handlers *)exception_handlers; - int vector = 0, ec; + int vector = 0; + unsigned long ec; ec = regs->cause & ~CAUSE_IRQ_FLAG; if (ec >= NR_EXCEPTIONS) @@ -432,6 +454,46 @@ void route_exception(struct pt_regs *regs) ec = 0; } + /* + * Handle V (vector) extension lazy enablement before any + * registered handler. The compiler's default march may include + * V, and auto-vectorization generates vector instructions that + * trigger EXC_INST_ILLEGAL when VS (Vector Status) in sstatus + * is Off. Enable VS to Initial and re-execute the faulting + * instruction, mimicking what a real OS kernel does. + * + * This check runs before any test-registered handler, so tests + * that install their own EXC_INST_ILLEGAL handler (e.g. + * sbi_pmu_test) are not affected. + */ + if (!(regs->cause & CAUSE_IRQ_FLAG) && ec == EXC_INST_ILLEGAL) { + /* + * If KVM supports the V extension for this Guest and VS + * (Vector Status) is Off in the saved sstatus, set it to + * Initial and re-execute the faulting instruction. + * + * Use regs->status (saved at exception entry) rather than + * reading the live CSR to avoid a TOCTOU race. + * + * Track the epc per-vCPU to avoid an infinite loop when + * V is disabled or the hardware rejects the VS change. + * Using a per-vCPU array avoids races between concurrent + * vCPUs that would occur with a single shared (epc, vcpu) + * pair. + */ + if (handlers && handlers->v_available && !(regs->status & SR_VS)) { + unsigned int vcpu_id; + + asm volatile("csrr %0, sscratch" : "=r" (vcpu_id)); + if (vcpu_id < handlers->v_epc_capacity && + handlers->v_epc[vcpu_id] != regs->epc) { + handlers->v_epc[vcpu_id] = regs->epc; + regs->status |= SR_VS_INITIAL; + return; + } + } + } + if (handlers && handlers->exception_handlers[vector][ec]) return handlers->exception_handlers[vector][ec](regs); @@ -448,9 +510,26 @@ void vcpu_init_vector_tables(struct kvm_vcpu *vcpu) void vm_init_vector_tables(struct kvm_vm *vm) { - vm->handlers = __vm_alloc(vm, sizeof(struct handlers), vm->page_size, + unsigned int max_vcpus; + size_t size; + + if (vm->handlers) + return; + + max_vcpus = kvm_check_cap(KVM_CAP_MAX_VCPUS); + if (max_vcpus == 0) + max_vcpus = 512; + + size = sizeof(struct handlers) + max_vcpus * sizeof(unsigned long); + vm->handlers = __vm_alloc(vm, size, vm->page_size, MEM_REGION_DATA); + { + struct handlers *h = addr_gva2hva(vm, vm->handlers); + + h->v_epc_capacity = max_vcpus; + } + *(gva_t *)addr_gva2hva(vm, (gva_t)(&exception_handlers)) = vm->handlers; } -- 2.43.0
