Subject: Re: [PATCH 3/6] RISC-V: KVM: Forward SBI DBCN extension to user-space

From: Greg Kroah-Hartman <[email-masked]>
Date: Tue, 10 Oct 2023 19:15:38 +0200

On Tue, Oct 10, 2023 at 10:35:00PM +0530, Anup Patel wrote:
> The SBI DBCN extension needs to be emulated in user-space

Why?

> so let
> us forward console_puts() call to user-space.

What could go wrong!

Why does userspace have to get involved in a console message?  Why is
this needed at all?  The kernel can not handle userspace consoles as
obviously they have to be re-entrant and irq safe.

> 
> Signed-off-by: Anup Patel <[email-masked]>
> ---
>  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_replace.c     | 31 +++++++++++++++++++++++++++
>  4 files changed, 37 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/kvm_vcpu_sbi.h b/arch/riscv/include/asm/kvm_vcpu_sbi.h
> index 8d6d4dce8a5e..a85f95eb6e85 100644
> --- a/arch/riscv/include/asm/kvm_vcpu_sbi.h
> +++ b/arch/riscv/include/asm/kvm_vcpu_sbi.h
> @@ -69,6 +69,7 @@ extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_ipi;
>  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_experimental;
>  extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_vendor;
>  
> diff --git a/arch/riscv/include/uapi/asm/kvm.h b/arch/riscv/include/uapi/asm/kvm.h
> index 917d8cc2489e..60d3b21dead7 100644
> --- a/arch/riscv/include/uapi/asm/kvm.h
> +++ b/arch/riscv/include/uapi/asm/kvm.h
> @@ -156,6 +156,7 @@ enum KVM_RISCV_SBI_EXT_ID {
>  	KVM_RISCV_SBI_EXT_PMU,
>  	KVM_RISCV_SBI_EXT_EXPERIMENTAL,
>  	KVM_RISCV_SBI_EXT_VENDOR,
> +	KVM_RISCV_SBI_EXT_DBCN,
>  	KVM_RISCV_SBI_EXT_MAX,

You just broke a user/kernel ABI here, why?

thanks,

greg k-h