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. Laurent
