Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions

[Peter Maydell]

On 8 July 2014 18:20, Al Viro <address@hidden> wrote:
> On Tue, Jul 08, 2014 at 05:33:16PM +0100, Peter Maydell wrote:
>
>> > Incidentally, combination of --enable-gprof and (default) --enable-pie
>> > won't build - it dies with ld(1) complaining about relocs in gcrt1.o.
>>
>> This sounds like a toolchain bug to me :-)
>
> Debian stable/amd64, gcc 4.7.2, binutils 2.22.  And google search finds
> this, for example: http://osdir.com/ml/qemu-devel/2013-05/msg00710.html.
> That one has gcc 4.4.3.

That just makes it a long-standing toolchain bug. I don't see any
reason why PIE + gprof shouldn't work, it just looks like gprof
doesn't ship and link a PIE runtime.

> Stats I quoted were from qemu-system-alpha booting debian/lenny (5.10) and
> going through their kernel package build.  I have perf report in front of
> me right now; the top ones are
>  41.77%  qemu-system-alp  perf-24701.map           [.] 0x7fbbee558930
>  11.78%  qemu-system-alp  qemu-system-alpha        [.] cpu_alpha_exec

> and cpu_alpha_exec() spends most of the time in inlined tb_find_fast().
> It might be worth checking the actual distribution of the hash of virt
> address used by that sucker - I wonder if dividing its argument by 4
> wouldn't improve the things, but I don't have stats on actual frequency
> of conflicts, etc.  In any case, the first lump (42%) seems to be tastier ;-)

Depends on your point of view -- arguably we ought to be spending *more*
time executing translated guest code... (As you say, the problem is that
we don't have any breakdown of what things might turn out to be hotspots
in the translated code.)

-- PMM