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

[Aurelien Jarno]

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.

> --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
> 
> 
> 
> 

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