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

[Anup Patel]

On Wed, Jan 25, 2023 at 7:53 PM Andy Chiu <andy.chiu at 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 at 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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Overall, this patch can be accepted independent of this series due
to its usefulness.

I send a v2 of this patch separately.

Regards,
Anup