| Message ID | 20230727081726.12650-1-nikita.shubin@maquefel.me (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] target/riscv: don't read write-only CSR | expand |
Patch looks correct IMHO. I have a few cosmetic suggestions: On 7/27/23 05:17, Nikita Shubin wrote: > From: Nikita Shubin <n.shubin@yadro.com> > > In case of write-only CSR don't return illegal inst error when CSR is > written and lacks read op. It's worth citing the ISA bits you put down below in the commit msg. It would also be nice to explain why checking for 'ret_value != NULL' is the right fix. In this case, both trans_csrrw() and trans_csrrwi() will check if rd=x0 and, in this case, will redirect to do_csrw(). do_csrw() will call riscv_csrrw_do64(), via helper_csrw(), with ret_val = NULL. And ... > > Signed-off-by: Nikita Shubin <n.shubin@yadro.com> > --- > Changelog: > - fixed uninitialized old_value > > Anyway it not might be a good idea to read CSR when we are not asked > for, during CSRRW or CSRRWI: > > "For CSRRWI, if rd=x0, then the instruction shall not read the CSR and > shall not cause any of the side effects that might occur on a CSR read." > > May be i am missing something of course. > --- > target/riscv/csr.c | 21 ++++++++++++--------- > 1 file changed, 12 insertions(+), 9 deletions(-) > > diff --git a/target/riscv/csr.c b/target/riscv/csr.c > index ea7585329e..3f0b3277e4 100644 > --- a/target/riscv/csr.c > +++ b/target/riscv/csr.c > @@ -3908,21 +3908,24 @@ static RISCVException riscv_csrrw_do64(CPURISCVState *env, int csrno, > target_ulong write_mask) > { > RISCVException ret; > - target_ulong old_value; > + target_ulong old_value = 0; > > /* execute combined read/write operation if it exists */ > if (csr_ops[csrno].op) { > return csr_ops[csrno].op(env, csrno, ret_value, new_value, write_mask); > } > > - /* if no accessor exists then return failure */ > - if (!csr_ops[csrno].read) { > - return RISCV_EXCP_ILLEGAL_INST; > - } > - /* read old value */ > - ret = csr_ops[csrno].read(env, csrno, &old_value); > - if (ret != RISCV_EXCP_NONE) { > - return ret; > + /* don't read if ret_value==NULL */ I would add in this comment that ret_value == NULL means that rd=x0 and we're coming from helper_csrw() and we can't throw side effects caused by CSR reads. Thanks, Daniel > + if (ret_value) { > + /* if no accessor exists then return failure */ > + if (!csr_ops[csrno].read) { > + return RISCV_EXCP_ILLEGAL_INST; > + } > + /* read old value */ > + ret = csr_ops[csrno].read(env, csrno, &old_value); > + if (ret != RISCV_EXCP_NONE) { > + return ret; > + } > } > > /* write value if writable and write mask set, otherwise drop writes */
diff --git a/target/riscv/csr.c b/target/riscv/csr.c index ea7585329e..3f0b3277e4 100644 --- a/target/riscv/csr.c +++ b/target/riscv/csr.c @@ -3908,21 +3908,24 @@ static RISCVException riscv_csrrw_do64(CPURISCVState *env, int csrno, target_ulong write_mask) { RISCVException ret; - target_ulong old_value; + target_ulong old_value = 0; /* execute combined read/write operation if it exists */ if (csr_ops[csrno].op) { return csr_ops[csrno].op(env, csrno, ret_value, new_value, write_mask); } - /* if no accessor exists then return failure */ - if (!csr_ops[csrno].read) { - return RISCV_EXCP_ILLEGAL_INST; - } - /* read old value */ - ret = csr_ops[csrno].read(env, csrno, &old_value); - if (ret != RISCV_EXCP_NONE) { - return ret; + /* don't read if ret_value==NULL */ + if (ret_value) { + /* if no accessor exists then return failure */ + if (!csr_ops[csrno].read) { + return RISCV_EXCP_ILLEGAL_INST; + } + /* read old value */ + ret = csr_ops[csrno].read(env, csrno, &old_value); + if (ret != RISCV_EXCP_NONE) { + return ret; + } } /* write value if writable and write mask set, otherwise drop writes */