On Mon, Dec 19, 2022 at 10:36:47PM -0800, Xin Li wrote:
> +static DEFINE_FRED_HANDLER(fred_exception)
> +{
> + /*
> + * This intentially omits exceptions that cannot happen on FRED h/w:
> + * vectors _NOT_ listed are set to NULL.
> + */
> + static const fred_handler exception_handlers[NUM_EXCEPTION_VECTORS] = {
> + [X86_TRAP_DE] = fred_exc_divide_error,
> + [X86_TRAP_DB] = fred_exc_debug,
> + [X86_TRAP_NMI] = NULL, /* A separate event type, not handled here */
> + [X86_TRAP_BP] = exc_int3,
> + [X86_TRAP_OF] = fred_exc_overflow,
> + [X86_TRAP_BR] = fred_exc_bounds,
> + [X86_TRAP_UD] = exc_invalid_op,
> + [X86_TRAP_NM] = fred_exc_device_not_available,
> + [X86_TRAP_DF] = fred_exc_double_fault,
> + [X86_TRAP_OLD_MF] = NULL, /* 387 only! */
> + [X86_TRAP_TS] = fred_exc_invalid_tss,
> + [X86_TRAP_NP] = fred_exc_segment_not_present,
> + [X86_TRAP_SS] = fred_exc_stack_segment,
> + [X86_TRAP_GP] = fred_exc_general_protection,
> + [X86_TRAP_PF] = fred_exc_page_fault,
> + [X86_TRAP_SPURIOUS] = NULL, /* Interrupts are their own event type */
> + [X86_TRAP_MF] = fred_exc_coprocessor_error,
> + [X86_TRAP_AC] = fred_exc_alignment_check,
> + [X86_TRAP_MC] = fred_exc_machine_check,
> + [X86_TRAP_XF] = fred_exc_simd_coprocessor_error
> + };
> + static const u32 noinstr_mask = BIT(X86_TRAP_DB) | BIT(X86_TRAP_BP) |
> + BIT(X86_TRAP_DF) | BIT(X86_TRAP_PF) |
> + BIT(X86_TRAP_MC) | BIT(X86_TRAP_UD);
> + u8 vector = array_index_nospec((u8)regs->vector, NUM_EXCEPTION_VECTORS);
> + irqentry_state_t state;
> +
> + if (likely(exception_handlers[vector])) {
Can't you get rid of this branch by stuffing the exception_handlers[]
table with fred_bad_event?
> + if (!(BIT(vector) & noinstr_mask)) {
> + state = irqentry_enter(regs);
> + instrumentation_begin();
> + }
> +
> + exception_handlers[vector](regs);
> +
> + if (!(BIT(vector) & noinstr_mask)) {
> + instrumentation_end();
> + irqentry_exit(regs, state);
> + }
This noinstr mask is daft; why not have DEFINE_FRED_HANDLER and
DEFINE_FRED_HANDLER_RAW or something, have the normal one include the
irqentry bits and use the _RAW one for the 'funny' ones that need to do
it themselves?
> + } else {
> + return fred_bad_event(regs);
> + }
Then all this becomes:
exception_handlers[vector](regs);
no branches, no nothing.
> +}
