target/riscv: Fix MEPC/SEPC bit masking for IALIGN

According to the RISC-V Privileged Architecture specification, the low
bit of MEPC/SEPC must always be zero. When IALIGN=32, the two low bits
must be zero.

This commit fixes the behavior of MEPC/SEPC CSR reads and writes, and
the implicit reads by MRET/SRET instructions to properly mask the
lowest bit(s) based on whether the C extension is enabled:
- When C extension is enabled (IALIGN=16): mask bit 0
- When C extension is disabled (IALIGN=32): mask bits [1:0]

Previously, when vectored mode bits from STVEC (which sets bit 0 for
vectored mode) were written to MEPC, the bits would not be cleared
correctly, causing incorrect behavior on MRET.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2855
Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.net>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250703182157.281320-2-charmitro@posteo.net>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

0 files changed, 0 insertions, 0 deletions