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. > --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 aurel...@aurel32.net http://www.aurel32.net
