Re: [PATCH v7 1/2] xen/riscv: introduce early_printk basic stuff

[Oleksii <oleksii.kurochko@gmail.com>]

Hi Alistair, Bobby and community,

I would like to ask your help with the following check:
+/*
+ * early_*() can be called from head.S with MMU-off.
+ *
+ * The following requiremets should be honoured for early_*() to
+ * work correctly:
+ *    It should use PC-relative addressing for accessing symbols.
+ *    To achieve that GCC cmodel=medany should be used.
+ */
+#ifndef __riscv_cmodel_medany
+#error "early_*() can be called from head.S with MMU-off"
+#endif

Please take a look at the following messages and help me to decide if
the check mentioned above should be in early_printk.c or not:
[1]
https://lore.kernel.org/xen-devel/599792fa-b08c-0b1e-10c1-0451519d9e4a@xen.org/
[2]
https://lore.kernel.org/xen-devel/0ec33871-96fa-bd9f-eb1b-eb37d3d7c982@xen.org/

Thanks in advance.

~ Oleksii

On Fri, 2023-01-27 at 13:39 +0200, Oleksii Kurochko wrote:
> Because printk() relies on a serial driver (like the ns16550 driver)
> and drivers require working virtual memory (ioremap()) there is not
> print functionality early in Xen boot.
> 
> The patch introduces the basic stuff of early_printk functionality
> which will be enough to print 'hello from C environment".
> 
> Originally early_printk.{c,h} was introduced by Bobby Eshleman
> (
> https://github.com/glg-rv/xen/commit/a3c9916bbdff7ad6030055bbee7e53d1a
> ab71384)
> but some functionality was changed:
> early_printk() function was changed in comparison with the original
> as
> common isn't being built now so there is no vscnprintf.
> 
> This commit adds early printk implementation built on the putc SBI
> call.
> 
> As sbi_console_putchar() is already being planned for deprecation
> it is used temporarily now and will be removed or reworked after
> real uart will be ready.
> 
> Signed-off-by: Bobby Eshleman <bobby.eshleman@gmail.com>
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
> Reviewed-by: Bobby Eshleman <bobby.eshleman@gmail.com>
> ---
> Changes in V7:
>     - Nothing was changed
> ---
> Changes in V6:
>     - Remove __riscv_cmodel_medany check from early_printk.c
> ---
> Changes in V5:
>     - Code style fixes
>     - Change an error message of #error in case of
> __riscv_cmodel_medany
>       isn't defined
> ---
> Changes in V4:
>     - Remove "depends on RISCV*" from Kconfig.debug as it is located
> in
>       arch specific folder so by default RISCV configs should be
> ebabled.
>     - Add "ifdef __riscv_cmodel_medany" to be sure that PC-relative
> addressing
>       is used as early_*() functions can be called from head.S with
> MMU-off and
>       before relocation (if it would be needed at all for RISC-V)
>     - fix code style
> ---
> Changes in V3:
>     - reorder headers in alphabetical order
>     - merge changes related to start_xen() function from "[PATCH v2
> 7/8]
>       xen/riscv: print hello message from C env" to this patch
>     - remove unneeded parentheses in definition of STACK_SIZE
> ---
> Changes in V2:
>     - introduce STACK_SIZE define.
>     - use consistent padding between instruction mnemonic and
> operand(s)
> ---
>  xen/arch/riscv/Kconfig.debug              |  5 ++++
>  xen/arch/riscv/Makefile                   |  1 +
>  xen/arch/riscv/early_printk.c             | 33
> +++++++++++++++++++++++
>  xen/arch/riscv/include/asm/early_printk.h | 12 +++++++++
>  xen/arch/riscv/setup.c                    |  4 +++
>  5 files changed, 55 insertions(+)
>  create mode 100644 xen/arch/riscv/early_printk.c
>  create mode 100644 xen/arch/riscv/include/asm/early_printk.h
> 
> diff --git a/xen/arch/riscv/Kconfig.debug
> b/xen/arch/riscv/Kconfig.debug
> index e69de29bb2..608c9ff832 100644
> --- a/xen/arch/riscv/Kconfig.debug
> +++ b/xen/arch/riscv/Kconfig.debug
> @@ -0,0 +1,5 @@
> +config EARLY_PRINTK
> +    bool "Enable early printk"
> +    default DEBUG
> +    help
> +      Enables early printk debug messages
> diff --git a/xen/arch/riscv/Makefile b/xen/arch/riscv/Makefile
> index fd916e1004..1a4f1a6015 100644
> --- a/xen/arch/riscv/Makefile
> +++ b/xen/arch/riscv/Makefile
> @@ -1,3 +1,4 @@
> +obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
>  obj-$(CONFIG_RISCV_64) += riscv64/
>  obj-y += sbi.o
>  obj-y += setup.o
> diff --git a/xen/arch/riscv/early_printk.c
> b/xen/arch/riscv/early_printk.c
> new file mode 100644
> index 0000000000..b66a11f1bc
> --- /dev/null
> +++ b/xen/arch/riscv/early_printk.c
> @@ -0,0 +1,33 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * RISC-V early printk using SBI
> + *
> + * Copyright (C) 2021 Bobby Eshleman <bobbyeshleman@gmail.com>
> + */
> +#include <asm/early_printk.h>
> +#include <asm/sbi.h>
> +
> +/*
> + * TODO:
> + *   sbi_console_putchar is already planned for deprecation
> + *   so it should be reworked to use UART directly.
> +*/
> +void early_puts(const char *s, size_t nr)
> +{
> +    while ( nr-- > 0 )
> +    {
> +        if ( *s == '\n' )
> +            sbi_console_putchar('\r');
> +        sbi_console_putchar(*s);
> +        s++;
> +    }
> +}
> +
> +void early_printk(const char *str)
> +{
> +    while ( *str )
> +    {
> +        early_puts(str, 1);
> +        str++;
> +    }
> +}
> diff --git a/xen/arch/riscv/include/asm/early_printk.h
> b/xen/arch/riscv/include/asm/early_printk.h
> new file mode 100644
> index 0000000000..05106e160d
> --- /dev/null
> +++ b/xen/arch/riscv/include/asm/early_printk.h
> @@ -0,0 +1,12 @@
> +#ifndef __EARLY_PRINTK_H__
> +#define __EARLY_PRINTK_H__
> +
> +#include <xen/early_printk.h>
> +
> +#ifdef CONFIG_EARLY_PRINTK
> +void early_printk(const char *str);
> +#else
> +static inline void early_printk(const char *s) {};
> +#endif
> +
> +#endif /* __EARLY_PRINTK_H__ */
> diff --git a/xen/arch/riscv/setup.c b/xen/arch/riscv/setup.c
> index 13e24e2fe1..d09ffe1454 100644
> --- a/xen/arch/riscv/setup.c
> +++ b/xen/arch/riscv/setup.c
> @@ -1,12 +1,16 @@
>  #include <xen/compile.h>
>  #include <xen/init.h>
>  
> +#include <asm/early_printk.h>
> +
>  /* Xen stack for bringing up the first CPU. */
>  unsigned char __initdata cpu0_boot_stack[STACK_SIZE]
>      __aligned(STACK_SIZE);
>  
>  void __init noreturn start_xen(void)
>  {
> +    early_printk("Hello from C env\n");
> +
>      for ( ;; )
>          asm volatile ("wfi");
>