Re: [PATCH] target/riscv/kvm: implement SBI debug console (DBCN) calls

[Andrew Jones]

On Thu, Apr 25, 2024 at 12:50:12PM GMT, Daniel Henrique Barboza wrote:
> SBI defines a Debug Console extension "DBCN" that will, in time, replace
> the legacy console putchar and getchar SBI extensions.
> 
> The appeal of the DBCN extension is that it allows multiple bytes to be
> read/written in the SBI console in a single SBI call.
> 
> As far as KVM goes, the DBCN calls are forwarded by an in-kernel KVM
> module to userspace. But this will only happens if the KVM module
> actually supports this SBI extension and we activate it.
> 
> We'll check for DBCN support during init time, checking if get-reg-list
> is advertising KVM_RISCV_SBI_EXT_DBCN. In that case, we'll enable it via
> kvm_set_one_reg() during kvm_arch_init_vcpu().
> 
> Finally, change kvm_riscv_handle_sbi() to handle the incoming calls for
> SBI_EXT_DBCN, reading and writing as required.
> 
> A simple KVM guest with 'earlycon=sbi', running in an emulated RISC-V
> host, takes around 20 seconds to boot without using DBCN. With this
> patch we're taking around 14 seconds to boot due to the speed-up in the
> terminal output.  There's no change in boot time if the guest isn't
> using earlycon.
> 
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> ---
>  target/riscv/kvm/kvm-cpu.c         | 111 +++++++++++++++++++++++++++++
>  target/riscv/sbi_ecall_interface.h |  17 +++++
>  2 files changed, 128 insertions(+)
>

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>