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

[Laurent Vivier <laurent@vivier.eu>]

Le 01/07/2015 15:15, Aurelien Jarno a écrit :
> On 2015-07-01 01:58, Laurent Vivier wrote:
>>
>>
>> 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)
> 
> Note that on x86_64, guest base is implemented by using the gs segment
> register. That explains why the impact should be relatively low, as your
> test shows.

I did a similar test on a PowerPC host, It is 2 seconds MORE on an 1m27s
build WITH --disable-guest-base.

So, definitively, I think the option can be dropped.

Laurent