Re: access guest address from within instruction

[Richard Henderson]

On 10/2/22 02:20, BitFriends wrote:
> I now came up with this code:
>
> TCGv_i64 res = 0;
> TCGv_i64 addr = (TCGv_i64)(env->regs[R_EDI]);
>
> tcg_gen_qemu_ld_i64(res, addr, 0, MO_LEUQ);
>
> env->regs[R_EAX] = (target_ulong)res;
>
> However this crashes afterwards in test_bit. Maybe this is caused by an invalid access? 
> Anything wrong about the code? This still gives some warnings, like TCGv_i32 expected (and 
> when you use TCGv_i32, it says TCGv_i64 expected) plus some casting warnings.

It is as if you did not read the second paragraph of my response at all.
tcg_gen_qemu_ld_i64 is for generating code, not performing a direct action.
Can you see how your code differs from *all* of the code around it?

r~

> Am Sa., 1. Okt. 2022 um 22:23 Uhr schrieb Richard Henderson <richard.henderson@linaro.org 
> <mailto:richard.henderson@linaro.org>>:
>
>     On 10/1/22 13:10, BitFriends wrote:
>      > Hello,
>      >
>      > I am trying to create a custom instruction that accesses guest memory 
> specified by an
>      > address in a register. I specifically want to read from that address. So 
> I tried to do
>      > that using "tcg_gen_qemu_ld_i64(&res, env->regs[R_EDI], 0, MO_LEUQ);", 
> but that
>     doesn't
>      > save any result in res.
>
>     This statement should have given you compilation errors, so I don't know 
> what you mean by
>     "doesn't save any result".  There's clearly a disconnect between what you 
> describe and
>     what you actually attempted.
>
>     Anyway, by the name you can see that function "gen"erates a "tcg" 
> operation, which is
>     then
>     later compiled by the jit, the output of which is later executed to produce 
> a result.
>     Which is, in general, what you want for implementing a custom instruction.
>
>
>     r~