Re: [Qemu-devel] [PATCH] linux-user: Avoid compilation error with --disable-guest-base

[Laurent Vivier <laurent@vivier.eu>]

Le 30/06/2015 19:20, Peter Maydell a écrit :
> On 30 June 2015 at 18:13, Laurent Vivier <laurent@vivier.eu> wrote:
>>
>>
>> Le 30/06/2015 18:45, Peter Maydell a écrit :
>>> On 30 June 2015 at 17:19, Laurent Vivier <laurent@vivier.eu> wrote:
>>>> When guest base is disabled, RESERVED_VA is 0, and
>>>> (__guest < RESERVED_VA) is always false as __guest is unsigned.
>>>>
>>>> With -Werror=type-limits, this triggers an error:
>>>>
>>>>     include/exec/cpu_ldst.h:60:31: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]
>>>>          (!RESERVED_VA || (__guest < RESERVED_VA)); \
>>>>
>>>> This patch removes this comparison when guest base is disabled.
>>>
>>> Is there a useful reason to compile with --disable-guest-base
>>> (ie why we should retain the !CONFIG_USE_GUEST_BASE code
>>> in QEMU at all) ? It was originally optional because we
>>> didn't support it in all our TCG hosts, but we fixed that
>>> back in 2012...
>>
>> TCG generates less code, so performance is better (well, it is what I
>> guess).
>>
>> I've compiled a kernel with and without guest base in a chrooted
>> linux-user-qemu.
>> Without guest base it is ~1 minute less for a 13 minutes build.
>>
>> I can do more tests if you want.
> 
> Hmm. That's a fair chunk of speedup. On the downside:
>  * you only get this if you're willing to build QEMU from
>    source with funny options
>  * it won't work for all guest/host combinations (sometimes
>    the guest really wants to be able to map at low addresses
>    the host won't permit)
>  * it's an extra configuration to maintain which we're
>    clearly not testing at all upstream
> 
> I'd still favour removing it completely, personally...

In fact, I have made more measurements, it saves only ~10 seconds on a
13 minutes build.

my test is: "make -j 4 vmlinux"
(target: m68k, host: x86_64, 4 cores x 2 threads)

--enable-guest-base

real    13m26.134s	13m28.712s	13m28.053s	13m28.875s
user    52m44.882s	52m56.075s	52m49.223s	52m55.366s
sys     0m33.452s	0m33.613s	0m33.013s	0m33.336s

--disable-guest-base

real    13m20.412s	13m17.773s	13m15.836s	13m13.278s
user    52m23.165s	52m7.184s	52m1.547s	51m50.277s
sys     0m33.427s	0m33.392s	0m32.954s	0m33.430s

Laurent