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Re: [Qemu-devel] [PATCH] linux-user: Avoid compilation error with --disa
From: |
Laurent Vivier |
Subject: |
Re: [Qemu-devel] [PATCH] linux-user: Avoid compilation error with --disable-guest-base |
Date: |
Wed, 1 Jul 2015 01:58:01 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.0.1 |
Le 30/06/2015 19:20, Peter Maydell a écrit :
> On 30 June 2015 at 18:13, Laurent Vivier <address@hidden> wrote:
>>
>>
>> Le 30/06/2015 18:45, Peter Maydell a écrit :
>>> On 30 June 2015 at 17:19, Laurent Vivier <address@hidden> 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