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

From: Andy Chiu <andy.chiu@sifive.com>
To: linux-riscv@lists.infradead.org, palmer@dabbelt.com,
	anup@brainfault.org, atishp@atishpatra.org,
	kvm-riscv@lists.infradead.org, kvm@vger.kernel.org
Cc: vineetg@rivosinc.com, greentime.hu@sifive.com,
	guoren@linux.alibaba.com, Andy Chiu <andy.chiu@sifive.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Albert Ou <aou@eecs.berkeley.edu>
Subject: [PATCH -next v13 18/19] riscv: kvm: redirect illegal instruction traps to guests
Date: Wed, 25 Jan 2023 14:20:55 +0000	[thread overview]
Message-ID: <20230125142056.18356-19-andy.chiu@sifive.com> (raw)
In-Reply-To: <20230125142056.18356-1-andy.chiu@sifive.com>

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


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