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) --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