target/riscv: Add textra matching condition for the triggers

According to RISC-V Debug specification, the optional textra32 and
textra64 trigger CSRs can be used to configure additional matching
conditions for the triggers. For example, if the textra.MHSELECT field
is set to 4 (mcontext), this trigger will only match or fire if the low
bits of mcontext/hcontext equal textra.MHVALUE field.

This commit adds the aforementioned matching condition as common trigger
matching conditions. Currently, the only legal values of textra.MHSELECT
are 0 (ignore) and 4 (mcontext). When textra.MHSELECT is 0, we pass the
checking. When textra.MHSELECT is 4, we compare textra.MHVALUE with
mcontext CSR. The remaining fields, such as textra.SBYTEMASK,
textra.SVALUE, and textra.SSELECT, are hardwired to zero for now. Thus,
we skip checking them here.

Signed-off-by: Alvin Chang <alvinga@andestech.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240826024657.262553-3-alvinga@andestech.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/debug.h

@@ -131,6 +131,9 @@ enum {
 #define ITRIGGER_VU           BIT(25)
 #define ITRIGGER_VS           BIT(26)
 
+#define MHSELECT_IGNORE       0
+#define MHSELECT_MCONTEXT     4
+
 bool tdata_available(CPURISCVState *env, int tdata_index);
 
 target_ulong tselect_csr_read(CPURISCVState *env);