[PATCH -next v13 18/19] riscv: kvm: redirect illegal instruction traps to guests

Andy Chiu
2023-01-25T14:23:32Z

Running below m-mode, an illegal instruction trap where m-mode could not
handle would be redirected back to s-mode. However, kvm running in hs-mode
terminates the vs-mode software when it receive such exception code.
Instead, it should redirect the trap back to vs-mode, and let vs-mode trap
handler decide the next step.

Besides, hs-mode should run transparently to vs-mode. So terminating
guest OS breaks assumption for the kernel running in vs-mode.

We use first-use trap to enable Vector for user space processes. This
means that the user process running in u- or vu- mode will take an
illegal instruction trap for the first time using V. Then the s- or vs-
mode kernel would allocate V for the process. Thus, we must redirect the
trap back to vs-mode in order to get the first-use trap working for guest
OSes here.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
---
arch/riscv/kvm/vcpu_exit.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff --git a/arch/riscv/kvm/vcpu_exit.c b/arch/riscv/kvm/vcpu_exit.c
index c9f741ab26f5..2a02cb750892 100644
--- a/arch/riscv/kvm/vcpu_exit.c
+++ b/arch/riscv/kvm/vcpu_exit.c
@@ -162,6 +162,16 @@ void kvm_riscv_vcpu_trap_redirect(struct kvm_vcpu *vcpu,
vcpu->arch.guest_context.sepc = csr_read(CSR_VSTVEC);
}

+static int vcpu_trap_redirect_vs(struct kvm_vcpu *vcpu,
+ struct kvm_cpu_trap *trap)
+{
+ /* set up trap handler and trap info when it gets back to vs */
+ kvm_riscv_vcpu_trap_redirect(vcpu, trap);
+ /* return to s-mode by setting vcpu's SPP */
+ vcpu->arch.guest_context.sstatus |= SR_SPP;
+ return 1;
+}
+
/*
* Return > 0 to return to guest, < 0 on error, 0 (and set exit_reason) on
* proper exit to userspace.
@@ -179,6 +189,10 @@ int kvm_riscv_vcpu_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
ret = -EFAULT;
run->exit_reason = KVM_EXIT_UNKNOWN;
switch (trap->scause) {
+ case EXC_INST_ILLEGAL:
+ if (vcpu->arch.guest_context.hstatus & HSTATUS_SPV)
+ ret = vcpu_trap_redirect_vs(vcpu, trap);
+ break;
case EXC_VIRTUAL_INST_FAULT:
if (vcpu->arch.guest_context.hstatus & HSTATUS_SPV)
ret = kvm_riscv_vcpu_virtual_insn(vcpu, run, trap);
@@ -206,6 +220,7 @@ int kvm_riscv_vcpu_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
vcpu->arch.guest_context.hstatus);
kvm_err("SCAUSE=0x%lx STVAL=0x%lx HTVAL=0x%lx HTINST=0x%lx\n",
trap->scause, trap->stval, trap->htval, trap->htinst);
+ asm volatile ("ebreak\n\t");
}

return ret;
--
2.17.1