Re: [PATCH v11 2/5] xen/riscv: introduce decode_cause() stuff

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.07.2024 17:31, Oleksii Kurochko wrote:
> The patch introduces stuff needed to decode a reason of an
> exception.
> 
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
> Acked-by: Alistair Francis <alistair.francis@xxxxxxx>
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Changes in V11:
>  - Nothing changed. Only rebase.
> ---
> Changes in V10:
>  - add Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Changes in V9:
>  - This patch was reverted as breaks both release and randconfig builds.
>    I don't see the failures now. ( probably it was because of printk usage
>    which was not ready at that moment ).
>  - drop inclusion of <asm/csr.h> and <asm/early_printk.h>
>  - add <asm/riscv_encoding.h> for CAUSE_* in decode_trap_cause().
> ---
> Changes in V8:
>   - fix typo in return string from decode_reserved_interrupt_cause
>   - add Acked-by: Alistair Francis <alistair.francis@xxxxxxx>
> ---
> Changes in V7:
>  - Nothing changed. Only rebase.
> ---
> Changes in V6:
>  - Remove usage of LINK_TO_LOAD() due to the MMU being enabled first.
>  - Change early_printk() to printk()
> ---
> Changes in V5:
>   - Remove <xen/error.h> from riscv/traps/c as nothing would require
>     inclusion.
>   - decode_reserved_interrupt_cause(), decode_interrupt_cause(), 
> decode_cause, do_unexpected_trap()
>     were made as static they are expected to be used only in traps.c
>   - use LINK_TO_LOAD() for addresses which can be linker time relative.
> ---
> Changes in V4:
>   - fix string in decode_reserved_interrupt_cause()
> ---
> Changes in V3:
>   - Nothing changed
> ---
> Changes in V2:
>   - Make decode_trap_cause() more optimization friendly.
>   - Merge the pathc which introduces do_unexpected_trap() to the current one.
> ---
>  xen/arch/riscv/traps.c | 80 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 79 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/riscv/traps.c b/xen/arch/riscv/traps.c
> index 5415cf8d90..37cec40dfa 100644
> --- a/xen/arch/riscv/traps.c
> +++ b/xen/arch/riscv/traps.c
> @@ -9,13 +9,91 @@
>  #include <xen/sched.h>
>  
>  #include <asm/processor.h>
> +#include <asm/riscv_encoding.h>
>  #include <asm/traps.h>
>  
> -void do_trap(struct cpu_user_regs *cpu_regs)
> +static const char *decode_trap_cause(unsigned long cause)
> +{
> +    static const char *const trap_causes[] = {
> +        [CAUSE_MISALIGNED_FETCH] = "Instruction Address Misaligned",
> +        [CAUSE_FETCH_ACCESS] = "Instruction Access Fault",
> +        [CAUSE_ILLEGAL_INSTRUCTION] = "Illegal Instruction",
> +        [CAUSE_BREAKPOINT] = "Breakpoint",
> +        [CAUSE_MISALIGNED_LOAD] = "Load Address Misaligned",
> +        [CAUSE_LOAD_ACCESS] = "Load Access Fault",
> +        [CAUSE_MISALIGNED_STORE] = "Store/AMO Address Misaligned",
> +        [CAUSE_STORE_ACCESS] = "Store/AMO Access Fault",
> +        [CAUSE_USER_ECALL] = "Environment Call from U-Mode",
> +        [CAUSE_SUPERVISOR_ECALL] = "Environment Call from S-Mode",
> +        [CAUSE_MACHINE_ECALL] = "Environment Call from M-Mode",
> +        [CAUSE_FETCH_PAGE_FAULT] = "Instruction Page Fault",
> +        [CAUSE_LOAD_PAGE_FAULT] = "Load Page Fault",
> +        [CAUSE_STORE_PAGE_FAULT] = "Store/AMO Page Fault",
> +        [CAUSE_FETCH_GUEST_PAGE_FAULT] = "Instruction Guest Page Fault",
> +        [CAUSE_LOAD_GUEST_PAGE_FAULT] = "Load Guest Page Fault",
> +        [CAUSE_VIRTUAL_INST_FAULT] = "Virtualized Instruction Fault",
> +        [CAUSE_STORE_GUEST_PAGE_FAULT] = "Guest Store/AMO Page Fault",
> +    };
> +
> +    if ( cause < ARRAY_SIZE(trap_causes) && trap_causes[cause] )
> +        return trap_causes[cause];
> +    return "UNKNOWN";
> +}

While I committed this as-is, two more points: First, as soon any any
such array access becomes potentially reachable because of guest
activity, it wants to use array_access_nospec() or alike. Second, and
somewhat related, it may be desirable to avoid such double accesses.
Hopefully the compiler will do that for you. But

    const char *res = cause < ARRAY_SIZE(trap_causes) ? trap_causes[cause]
                                                      : NULL;

    return res ?: "UNKNOWN";

might be worthwhile anyway.

Jan