target/riscv: allow MISA writes as experimental
At this moment, and apparently since ever, we have no way of enabling RISCV_FEATURE_MISA. This means that all the code from write_misa(), all the nuts and bolts that handles how to properly write this CSR, has always been a no-op as well because write_misa() will always exit earlier. This seems to be benign in the majority of cases. Booting an Ubuntu 'virt' guest and logging all the calls to 'write_misa' shows that no writes to MISA CSR was attempted. Writing MISA, i.e. enabling/disabling RISC-V extensions after the machine is powered on, seems to be a niche use. After discussions in the mailing list, most notably in [1], we reached the consensus that this code is not suited to be exposed to users because it's not well tested, but at the same time removing it is a bit extreme because we would like to fix it, and it's easier to do so with the code available to use instead of fetching it from git log. The approach taken here is to get rid of RISCV_FEATURE_MISA altogether and use a new experimental flag called x-misa-w. The default value is false, meaning that we're keeping the existing behavior of doing nothing if a write_misa() is attempted. As with any existing experimental flag, x-misa-w is also a temporary flag that we need to remove once we fix write_misa(). [1] https://lists.gnu.org/archive/html/qemu-devel/2023-02/msg05092.html Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com> Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn> Reviewed-by: Andrew Jones <ajones@ventanamicro.com> Reviewed-by: Bin Meng <bmeng@tinylab.org> Reviewed-by: Weiwei Li<liweiwei@iscas.ac.cn> Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com> Message-ID: <20230222185205.355361-4-dbarboza@ventanamicro.com> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
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@ -1210,6 +1210,12 @@ static Property riscv_cpu_properties[] = {
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DEFINE_PROP_BOOL("rvv_ta_all_1s", RISCVCPU, cfg.rvv_ta_all_1s, false),
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DEFINE_PROP_BOOL("rvv_ma_all_1s", RISCVCPU, cfg.rvv_ma_all_1s, false),
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/*
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* write_misa() is marked as experimental for now so mark
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* it with -x and default to 'false'.
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*/
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DEFINE_PROP_BOOL("x-misa-w", RISCVCPU, cfg.misa_w, false),
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DEFINE_PROP_END_OF_LIST(),
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};
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@ -89,7 +89,6 @@ enum {
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RISCV_FEATURE_MMU,
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RISCV_FEATURE_PMP,
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RISCV_FEATURE_EPMP,
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RISCV_FEATURE_MISA,
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RISCV_FEATURE_DEBUG
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};
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@ -498,6 +497,7 @@ struct RISCVCPUConfig {
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bool pmp;
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bool epmp;
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bool debug;
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bool misa_w;
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bool short_isa_string;
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};
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@ -1329,7 +1329,7 @@ static RISCVException read_misa(CPURISCVState *env, int csrno,
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static RISCVException write_misa(CPURISCVState *env, int csrno,
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target_ulong val)
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{
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if (!riscv_feature(env, RISCV_FEATURE_MISA)) {
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if (!riscv_cpu_cfg(env)->misa_w) {
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/* drop write to misa */
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return RISCV_EXCP_NONE;
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}
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