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

Anup Patel
2023-01-27T11:28:41Z

On Wed, Jan 25, 2023 at 7:53 PM Andy Chiu <andy.chiu@sifive.com> wrote:
>
> 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.

In general, it is a good strategy to always redirect illegal instruction
traps to VS-mode.

>
> 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;

Setting sstatus.SPP needs to be done in kvm_riscv_vcpu_trap_redirect()
because for guest all traps are always taken by VS-mode.

> + 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");

This is not a related change.

> }
>
> return ret;
> --
> 2.17.1
>
>
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> linux-riscv@lists.infradead.org
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Overall, this patch can be accepted independent of this series due
to its usefulness.

I send a v2 of this patch separately.

Regards,
Anup