[PATCH] RISC-V: KVM: Add SBI DBTR extension forwarding

[Anup Patel]

On Tue, Jul 21, 2026 at 8:12 AM Xie Bo <xb at ultrarisc.com> wrote:
>
> The SBI headers define the Debug Triggers extension and its function IDs,
> but KVM does not currently allow a VMM to expose DBTR to a guest.
>
> Add DBTR to the KVM SBI extension list and forward its calls to userspace
> through the existing KVM_EXIT_RISCV_SBI path. Keep the extension disabled
> by default so that userspace must explicitly enable it through the SBI
> extension ONE_REG interface before running the VCPU.

Blindly forwarding SBI DBTR calls to KVM user-space will not work because
(just like SBI PMU) we will be sharing HW debug triggers between host and
guest where KVM RISC-V will have a separate path through the perf subsystem
to configure debug triggers for guests.

We (Qualcomm) are already working on a series for SBI DBTR virtualization
in KVM RISC-V so stay tuned.

Regards,
Anup

>
> Signed-off-by: Xie Bo <xb at ultrarisc.com>
> ---
>  arch/riscv/include/asm/kvm_vcpu_sbi.h | 1 +
>  arch/riscv/include/uapi/asm/kvm.h     | 1 +
>  arch/riscv/kvm/vcpu_sbi.c             | 4 ++++
>  arch/riscv/kvm/vcpu_sbi_forward.c     | 7 +++++++
>  4 files changed, 13 insertions(+)
>
> diff --git a/arch/riscv/include/asm/kvm_vcpu_sbi.h b/arch/riscv/include/asm/kvm_vcpu_sbi.h
> index c1a7e3b40..53406b02f 100644
> --- a/arch/riscv/include/asm/kvm_vcpu_sbi.h
> +++ b/arch/riscv/include/asm/kvm_vcpu_sbi.h
> @@ -104,6 +104,7 @@ extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_rfence;
>  extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_srst;
>  extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_hsm;
>  extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_dbcn;
> +extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_dbtr;
>  extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_susp;
>  extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_sta;
>  extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_fwft;
> diff --git a/arch/riscv/include/uapi/asm/kvm.h b/arch/riscv/include/uapi/asm/kvm.h
> index 504e73305..047d82f05 100644
> --- a/arch/riscv/include/uapi/asm/kvm.h
> +++ b/arch/riscv/include/uapi/asm/kvm.h
> @@ -221,6 +221,7 @@ enum KVM_RISCV_SBI_EXT_ID {
>         KVM_RISCV_SBI_EXT_SUSP,
>         KVM_RISCV_SBI_EXT_FWFT,
>         KVM_RISCV_SBI_EXT_MPXY,
> +       KVM_RISCV_SBI_EXT_DBTR,
>         KVM_RISCV_SBI_EXT_MAX,
>  };
>
> diff --git a/arch/riscv/kvm/vcpu_sbi.c b/arch/riscv/kvm/vcpu_sbi.c
> index 46ab7b989..ed21239c5 100644
> --- a/arch/riscv/kvm/vcpu_sbi.c
> +++ b/arch/riscv/kvm/vcpu_sbi.c
> @@ -70,6 +70,10 @@ static const struct kvm_riscv_sbi_extension_entry sbi_ext[] = {
>                 .ext_idx = KVM_RISCV_SBI_EXT_DBCN,
>                 .ext_ptr = &vcpu_sbi_ext_dbcn,
>         },
> +       {
> +               .ext_idx = KVM_RISCV_SBI_EXT_DBTR,
> +               .ext_ptr = &vcpu_sbi_ext_dbtr,
> +       },
>         {
>                 .ext_idx = KVM_RISCV_SBI_EXT_SUSP,
>                 .ext_ptr = &vcpu_sbi_ext_susp,
> diff --git a/arch/riscv/kvm/vcpu_sbi_forward.c b/arch/riscv/kvm/vcpu_sbi_forward.c
> index 5a3c75eb2..8ac31b53f 100644
> --- a/arch/riscv/kvm/vcpu_sbi_forward.c
> +++ b/arch/riscv/kvm/vcpu_sbi_forward.c
> @@ -26,6 +26,13 @@ const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_dbcn = {
>         .handler = kvm_riscv_vcpu_sbi_forward_handler,
>  };
>
> +const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_dbtr = {
> +       .extid_start = SBI_EXT_DBTR,
> +       .extid_end = SBI_EXT_DBTR,
> +       .default_disabled = true,
> +       .handler = kvm_riscv_vcpu_sbi_forward_handler,
> +};
> +
>  const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_mpxy = {
>         .extid_start = SBI_EXT_MPXY,
>         .extid_end = SBI_EXT_MPXY,
>
> base-commit: b95f03f04d475aa6719d15a636ddf32222d55657
> --
> 2.54.0
>