Re: [PATCH 1/3] accel/tcg: Set can_do_io at at start of lookup_tb_ptr helper

[Richard Henderson <richard.henderson@linaro.org>]

On 2/15/24 05:01, Jonathan Cameron wrote:
> From: Peter Maydell <peter.maydell@linaro.org>
> 
> Peter posted this in the thread trying to fix x86 TCG handling
> of page tables in MMIO space (specifically emulated CXL interleaved memory)
> https://lore.kernel.org/qemu-devel/CAFEAcA_a_AyQ=Epz3_+CheAT8Crsk9mOu894wbNW_FywamkZiw@mail.gmail.com/#t
> 
> Peter, are you happy to give your SoB on this one?
> 
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> ---
>   accel/tcg/cpu-exec.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index 977576ca14..52239a441f 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -396,6 +396,14 @@ const void *HELPER(lookup_tb_ptr)(CPUArchState *env)
>       uint64_t cs_base;
>       uint32_t flags, cflags;
>   
> +    /*
> +     * By definition we've just finished a TB, so I/O is OK.
> +     * Avoid the possibility of calling cpu_io_recompile() if
> +     * a page table walk triggered by tb_lookup() calling
> +     * probe_access_internal() happens to touch an MMIO device.
> +     * The next TB, if we chain to it, will clear the flag again.
> +     */
> +    cpu->neg.can_do_io = true;

Yes, this mirrors what is done in cpu_loop_exit(), and I can see how get_page_addr_code() 
and the ptw within would require can_do_io set properly.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~