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>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20240425155012.581366-1-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
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