Re: [PATCH] accel/tcg: remove link between guest ram and TCG cache size

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

On 2/26/20 7:27 AM, Alex Bennée wrote:
> Basing the TB cache size on the ram_size was always a little heuristic
> and was broken by a1b18df9a4 which caused ram_size not to be fully
> realised at the time we initialise the TCG translation cache.
> 
> At the same time the default code generation size seems mainly set to
> deal with the fact we use a static code buffer for CONFIG_USER to
> avoid mmap allocation problems on constrained systems. So we:
> 
>   - only use a static code buffer on 32 bit systems
>   - up the default buffer size for bigger systems
>   - ignore the ram_size and just go with the default
>   - document the fact tb-size is ignored for 32 bit linux-user
> 
> The could potentially slow down softmmu emulation on 32 bit systems
> with lots (3gb?) of spare memory. Those users can still manually up
> the tb-size via the command line if they do in fact exist.
> 
> Fixes: a1b18df9a4
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Cc: Niek Linnenbank <nieklinnenbank@gmail.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> ---
...
> -#if defined(CONFIG_USER_ONLY)
> -/* Currently it is not recommended to allocate big chunks of data in
> -   user mode. It will change when a dedicated libc will be used.  */
> -/* ??? 64-bit hosts ought to have no problem mmaping data outside the
> -   region in which the guest needs to run.  Revisit this.  */
> +#if defined(CONFIG_USER_ONLY) && TCG_TARGET_REG_BITS == 32
> +/*
> + * For user mode on smaller 32 bit systems we may run into trouble
> + * allocating big chunks of data in the right place. On these systems
> + * we utilise a static code generation buffer directly in the binary.
> + */
>  #define USE_STATIC_CODE_GEN_BUFFER
>  #endif
>  
> @@ -927,7 +928,11 @@ static void page_lock_pair(PageDesc **ret_p1, tb_page_addr_t phys1,
>  # define MAX_CODE_GEN_BUFFER_SIZE  ((size_t)-1)
>  #endif
>  
> +#if TCG_TARGET_REG_BITS == 32
>  #define DEFAULT_CODE_GEN_BUFFER_SIZE_1 (32u * 1024 * 1024)
> +#else
> +#define DEFAULT_CODE_GEN_BUFFER_SIZE_1 (2ul * 1024 * 1024 * 1024)
> +#endif

As Igor notes, there is no -tb-size X or -accel tcg,tb-size=Y option for
linux-user.  Therefore I'd prefer this patch merely fix the default for softmmu
and not change linux-user at all.

I agree it is somewhat silly to use the static code gen buffer on a 64-bit
host, but let's leave that to a separate patch.


r~