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Re: [Qemu-devel] [PATCH] linux-user: Avoid compilation error with --disa


From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH] linux-user: Avoid compilation error with --disable-guest-base
Date: Wed, 1 Jul 2015 12:12:33 +0100

On 1 July 2015 at 00:58, Laurent Vivier <address@hidden> wrote:
> 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.

...that seems like it swings the argument pretty strongly
towards dropping the no-guest-base config entirely.

I think my suggestion would be:
 1) apply this patch for 2.4
 2) note in the 2.4 changelog that we're planning to drop
   --disable-guest-base for 2.5
 3) remove the config option and the code after 2.4 releases

thanks
-- PMM



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