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

[Aurelien Jarno]

On 2015-07-01 20:21, Laurent Vivier wrote:
> 
> 
> 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 <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)
> > 
> > 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.

I fully agree.

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
address@hidden                 http://www.aurel32.net