Re: [PATCH v1 4/8] xen/riscv: introduce sbi call to putchar to console

[Bobby Eshleman <bobbyeshleman@xxxxxxxxx>]

On Fri, Jan 06, 2023 at 03:14:25PM +0200, Oleksii Kurochko wrote:
> The patch introduce sbi_putchar() SBI call which is necessary
> to implement initial early_printk
> 

I think that it might be wise to start off with an alternative to
sbi_putchar() since it is already planned for deprecation. I realize
that this will require rework, but it is almost guaranteed that
early_printk() will break on future SBI implementations if using this
SBI call. IIRC, Xen/ARM's early printk looked like a reasonable analogy
for how it could work on RISC-V, IMHO.

> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
> ---
>  xen/arch/riscv/Makefile          |  1 +
>  xen/arch/riscv/include/asm/sbi.h | 34 ++++++++++++++++++++++++
>  xen/arch/riscv/sbi.c             | 44 ++++++++++++++++++++++++++++++++
>  3 files changed, 79 insertions(+)
>  create mode 100644 xen/arch/riscv/include/asm/sbi.h
>  create mode 100644 xen/arch/riscv/sbi.c
> 
> diff --git a/xen/arch/riscv/Makefile b/xen/arch/riscv/Makefile
> index 5a67a3f493..60db415654 100644
> --- a/xen/arch/riscv/Makefile
> +++ b/xen/arch/riscv/Makefile
> @@ -1,5 +1,6 @@
>  obj-$(CONFIG_RISCV_64) += riscv64/
>  obj-y += setup.o
> +obj-y += sbi.o
>  
>  $(TARGET): $(TARGET)-syms
>       $(OBJCOPY) -O binary -S $< $@
> diff --git a/xen/arch/riscv/include/asm/sbi.h 
> b/xen/arch/riscv/include/asm/sbi.h
> new file mode 100644
> index 0000000000..34b53f8eaf
> --- /dev/null
> +++ b/xen/arch/riscv/include/asm/sbi.h
> @@ -0,0 +1,34 @@
> +/* SPDX-License-Identifier: (GPL-2.0-or-later) */
> +/*
> + * Copyright (c) 2021 Vates SAS.
> + *
> + * Taken from xvisor, modified by Bobby Eshleman (bobby.eshleman@xxxxxxxxx).
> + *
> + * Taken/modified from Xvisor project with the following copyright:
> + *
> + * Copyright (c) 2019 Western Digital Corporation or its affiliates.
> + */
> +
> +#ifndef __CPU_SBI_H__
> +#define __CPU_SBI_H__
> +
> +#define SBI_EXT_0_1_CONSOLE_PUTCHAR          0x1
> +
> +struct sbiret {
> +    long error;
> +    long value;
> +};
> +
> +struct sbiret sbi_ecall(unsigned long ext, unsigned long fid, unsigned long 
> arg0,
> +        unsigned long arg1, unsigned long arg2,
> +        unsigned long arg3, unsigned long arg4,
> +        unsigned long arg5);
> +
> +/**
> + * Writes given character to the console device.
> + *
> + * @param ch The data to be written to the console.
> + */
> +void sbi_console_putchar(int ch);
> +
> +#endif // __CPU_SBI_H__
> diff --git a/xen/arch/riscv/sbi.c b/xen/arch/riscv/sbi.c
> new file mode 100644
> index 0000000000..67cf5dd982
> --- /dev/null
> +++ b/xen/arch/riscv/sbi.c
> @@ -0,0 +1,44 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +/*
> + * Taken and modified from the xvisor project with the copyright Copyright 
> (c)
> + * 2019 Western Digital Corporation or its affiliates and author Anup Patel
> + * (anup.patel@xxxxxxx).
> + *
> + * Modified by Bobby Eshleman (bobby.eshleman@xxxxxxxxx).
> + *
> + * Copyright (c) 2019 Western Digital Corporation or its affiliates.
> + * Copyright (c) 2021 Vates SAS.
> + */
> +
> +#include <xen/errno.h>
> +#include <asm/sbi.h>
> +
> +struct sbiret sbi_ecall(unsigned long ext, unsigned long fid, unsigned long 
> arg0,
> +            unsigned long arg1, unsigned long arg2,
> +            unsigned long arg3, unsigned long arg4,
> +            unsigned long arg5)
> +{
> +    struct sbiret ret;
> +    register unsigned long a0 asm ("a0") = arg0;
> +    register unsigned long a1 asm ("a1") = arg1;
> +    register unsigned long a2 asm ("a2") = arg2;
> +    register unsigned long a3 asm ("a3") = arg3;
> +    register unsigned long a4 asm ("a4") = arg4;
> +    register unsigned long a5 asm ("a5") = arg5;
> +    register unsigned long a6 asm ("a6") = fid;
> +    register unsigned long a7 asm ("a7") = ext;
> +
> +    asm volatile ("ecall"
> +              : "+r" (a0), "+r" (a1)
> +              : "r" (a2), "r" (a3), "r" (a4), "r" (a5), "r" (a6), "r" (a7)
> +              : "memory");
> +    ret.error = a0;
> +    ret.value = a1;
> +
> +    return ret;
> +}
> +
> +void sbi_console_putchar(int ch)
> +{
> +    sbi_ecall(SBI_EXT_0_1_CONSOLE_PUTCHAR, 0, ch, 0, 0, 0, 0, 0);
> +}
> -- 
> 2.38.1
> 
>