Subject: Re: [PATCH 2/6] RISC-V: KVM: Change the SBI specification version to v2.0

From: Greg Kroah-Hartman <[email-masked]>
Date: Wed, 11 Oct 2023 17:26:06 +0200

On Wed, Oct 11, 2023 at 04:32:22PM +0530, Anup Patel wrote:
> On Wed, Oct 11, 2023 at 12:57 PM Greg Kroah-Hartman
> <[email-masked]> wrote:
> >
> > On Wed, Oct 11, 2023 at 11:49:14AM +0530, Anup Patel wrote:
> > > On Tue, Oct 10, 2023 at 10:43 PM Greg Kroah-Hartman
> > > <[email-masked]> wrote:
> > > >
> > > > On Tue, Oct 10, 2023 at 10:34:59PM +0530, Anup Patel wrote:
> > > > > We will be implementing SBI DBCN extension for KVM RISC-V so let
> > > > > us change the KVM RISC-V SBI specification version to v2.0.
> > > > >
> > > > > Signed-off-by: Anup Patel <[email-masked]>
> > > > > ---
> > > > >  arch/riscv/include/asm/kvm_vcpu_sbi.h | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/arch/riscv/include/asm/kvm_vcpu_sbi.h b/arch/riscv/include/asm/kvm_vcpu_sbi.h
> > > > > index cdcf0ff07be7..8d6d4dce8a5e 100644
> > > > > --- a/arch/riscv/include/asm/kvm_vcpu_sbi.h
> > > > > +++ b/arch/riscv/include/asm/kvm_vcpu_sbi.h
> > > > > @@ -11,7 +11,7 @@
> > > > >
> > > > >  #define KVM_SBI_IMPID 3
> > > > >
> > > > > -#define KVM_SBI_VERSION_MAJOR 1
> > > > > +#define KVM_SBI_VERSION_MAJOR 2
> > > >
> > > > What does this number mean?  Who checks it?  Why do you have to keep
> > > > incrementing it?
> > >
> > > This number is the SBI specification version implemented by KVM RISC-V
> > > for the Guest kernel.
> > >
> > > The original sbi_console_putchar() and sbi_console_getchar() are legacy
> > > functions (aka SBI v0.1) which were introduced a few years back along
> > > with the Linux RISC-V port.
> > >
> > > The latest SBI v2.0 specification (which is now frozen) introduces a new
> > > SBI debug console extension which replaces legacy sbi_console_putchar()
> > > and sbi_console_getchar() functions with better alternatives.
> > > (Refer, https://github.com/riscv-non-isa/riscv-sbi-doc/releases/download/commit-fe4562532a9cc57e5743b6466946c5e5c98c73ca/riscv-sbi.pdf)
> > >
> > > This series adds SBI debug console implementation in KVM RISC-V
> > > so the SBI specification version advertised by KVM RISC-V must also be
> > > upgraded to v2.0.
> > >
> > > Regarding who checks its, the SBI client drivers in the Linux kernel
> > > will check SBI specification version implemented by higher privilege
> > > mode (M-mode firmware or HS-mode hypervisor) before probing
> > > the SBI extension. For example, the HVC SBI driver (PATCH5)
> > > will ensure SBI spec version to be at least v2.0 before probing
> > > SBI debug console extension.
> >
> > Is this api backwards compatible, or did you just break existing
> > userspace that only expects version 1.0?
> 
> The legacy sbi_console_putchar() and sbi_console_getchar()
> functions have not changed so it does not break existing
> user-space.
> 
> The new SBI DBCN functions to be implemented by KVM
> user space are:
> sbi_debug_console_write()
> sbi_debug_console_read()
> sbi_debug_console_write_byte()

And where exactly is that code for us to review that this is tested?

thanks,

greg k-h