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

["Alex Bennée" <alex.bennee@linaro.org>]

Richard Henderson <richard.henderson@linaro.org> writes:

> 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.

I don't disagree.

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

OK I'll have a look at that.

> (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?

Some random thoughts in no particular order:

 - a separately flushable translation region for code we detect as SMC heavy

 - a front-end interpreter for SMC code

 - smarter code generation that dynamically loads values from codemem
   (usually the SMC code is just tweaking an #imm value)

None of these seem particularly amenable to a clean non-complex
implementation though. A front-end interpreter would be useful for other
things though - it could even be incomplete and handle only common code
patterns falling back to full generation for anything it can't handle.

>
>
> r~


-- 
Alex Bennée