Re: [RFC PATCH] accel/tcg: change default codegen buffer size for i386-softmmu

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

On 5/25/21 9:45 AM, Alex Bennée wrote:
> There are two justifications for making this change. The first is that
> i386 emulation is typically for smaller machines where having a 1gb of
> generated code is overkill for basic emulation. The second is the
> propensity of self-modifying code (c.f. Doom/edit) utilised on i386
> systems can trigger a rapid growth in invalidated and re-translated
> buffers. This is seen in bug #283. Execution is still inefficient but
> at least the host memory isn't so aggressively used up.
> 
> That said it's still really just a sticking plaster for user
> convenience.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Cc: Thomas Huth <thuth@redhat.com>
> Cc: 1896298@bugs.launchpad.net
> ---
>   accel/tcg/translate-all.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> index 640ff6e3e7..f442165674 100644
> --- a/accel/tcg/translate-all.c
> +++ b/accel/tcg/translate-all.c
> @@ -951,9 +951,13 @@ static void page_lock_pair(PageDesc **ret_p1, tb_page_addr_t phys1,
>    * Users running large scale system emulation may want to tweak their
>    * runtime setup via the tb-size control on the command line.
>    */
> +#ifdef TARGET_I386
> +#define DEFAULT_CODE_GEN_BUFFER_SIZE_1 (32 * MiB)
> +#else
>   #define DEFAULT_CODE_GEN_BUFFER_SIZE_1 (1 * GiB)
>   #endif
>   #endif
> +#endif
>   
>   #define DEFAULT_CODE_GEN_BUFFER_SIZE \
>     (DEFAULT_CODE_GEN_BUFFER_SIZE_1 < MAX_CODE_GEN_BUFFER_SIZE \
> 

I'm not thrilled, as it is ultra-hacky.

(1) I've got a re-org of this code out for review: 
https://patchew.org/QEMU/20210502231844.1977630-1-richard.henderson@linaro.org/

(2) I'm keen to reorg TCG such that it gets compiled once.  There's currently 
nothing standing in the way of that except work.  But this would introduce a 
use of a target-specific define for the first time into tcg/.  I guess I could 
leave the default sizing back in accel/tcg/ and pass in the default.

Other options?


r~