target/riscv: throw debug exception before page fault

target/riscv: throw debug exception before page fault

In the RISC-V privileged ISA section 3.1.15 table 15, it is determined
that a debug exception that is triggered from a load/store has a higher
priority than a possible fault that this access might trigger.

This is not the case ATM as shown in [1]. Adding a breakpoint in an
address that deliberately will fault is causing a load page fault
instead of a debug exception. The reason is that we're throwing in the
page fault as soon as the fault occurs (end of riscv_cpu_tlb_fill(),
raise_mmu_exception()), not allowing the installed watchpoints to
trigger.

Call cpu_check_watchpoint() in the page fault path to search and execute
any watchpoints that might exist for the address, never returning back
to the fault path. If no watchpoints are found cpu_check_watchpoint()
will return and we'll fall-through the regular path to
raise_mmu_exception().

[1] https://gitlab.com/qemu-project/qemu/-/issues/2627

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2627
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250121170626.1992570-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>