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Re: [Qemu-devel] [PATCH] linux-user: Avoid compilation error with --disa
Re: [Qemu-devel] [PATCH] linux-user: Avoid compilation error with --disable-guest-base
Wed, 1 Jul 2015 15:15:36 +0200
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
> 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
> 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
Aurelien Jarno GPG: 4096R/1DDD8C9B
- Re: [Qemu-devel] [PATCH] linux-user: Avoid compilation error with --disable-guest-base,
Aurelien Jarno <=