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id = 2855
title = "masking mode field in mepc before mret"
state = "opened"
created_at = "2025-03-06T09:38:42.439Z"
closed_at = "n/a"
labels = ["accel: TCG", "kind::Bug", "target: riscv"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/2855"
host-os = "all"
host-arch = "riscv"
qemu-version = "QEMU emulator version 9.2.0 (v9.2.0)"
guest-os = "qemu-system-riscv64"
guest-arch = "riscv"
description = """I thought I found a bug in OpenSBI (https://github.com/riscv-software-src/opensbi/issues/391) but it actually is a QEMU bug.
It is described here: https://lists.infradead.org/pipermail/opensbi/2025-March/008166.html"""
reproduce = """1. use an application with vectored mode enabled (The RISC-V Instruction Set Manual: Volume II: Privileged Architecture / chapter 10.1.2) in QEMU
2. trigger an illegal instruction interrupt (handle it in machine mode - not by medeleg)
3. in a machine mode trap: Store STVEC in MEPC.
4. do a mret
5. the first bits of mepc are not masked so the address in mepc (comming from (v)stvec) will be false after mret"""
additional = """My guess is that the instructions from the following quote (masking of lower bits in mepc) from the official spec must be implemented here:
https://gitlab.com/qemu-project/qemu/-/blob/master/target/riscv/op_helper.c?ref_type=heads#L387
Maybe also somewhere else.
> 3.1.14. Machine Exception Program Counter (mepc)
>
> mepc is an MXLEN-bit read/write register formatted as shown in Figure 21. The low bit of mepc
> (mepc[0]) is always zero. On implementations that support only IALIGN=32, the two low bits
> (mepc[1:0]) are always zero.
>
> If an implementation allows IALIGN to be either 16 or 32 (by changing CSR misa, for example), then,
> whenever IALIGN=32, bit mepc[1] is masked on reads so that it appears to be 0. This masking occurs
> also for the implicit read by the MRET instruction. Though masked, mepc[1] remains writable when
> IALIGN=32.
>
> mepc is a WARL register that must be able to hold all valid virtual addresses. It need not be capable of
> holding all possible invalid addresses. Prior to writing mepc, implementations may convert an invalid
> address into some other invalid address that mepc is capable of holding."""
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