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 <laur...@vivier.eu> wrote: > >>>> > >>>> > >>>> Le 30/06/2015 18:45, Peter Maydell a écrit : > >>>>> On 30 June 2015 at 17:19, Laurent Vivier <laur...@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.
I fully agree. -- Aurelien Jarno GPG: 4096R/1DDD8C9B aurel...@aurel32.net http://www.aurel32.net
