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

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Message-ID: <CAK9=C2XYQ0U9CbuCg6cTf79sSsy+0BxF5mBE0R+E3s9iZFzEWw@mail.gmail.com>
Date:   Wed, 11 Oct 2023 16:32:22 +0530
From:   Anup Patel <apatel@...tanamicro.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Atish Patra <atishp@...shpatra.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Conor Dooley <conor@...nel.org>,
        Andrew Jones <ajones@...tanamicro.com>, kvm@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-serial@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] RISC-V: KVM: Change the SBI specification version to v2.0

On Wed, Oct 11, 2023 at 12:57 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> 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
> > <gregkh@...uxfoundation.org> 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 <apatel@...tanamicro.com>
> > > > ---
> > > >  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()

>
> thanks,
>
> greg k-h

Regards,
Anup