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

["Alex Bennée" <alex.bennee@linaro.org>]

Al Viro writes:

> On Tue, Jul 08, 2014 at 08:32:55PM +0100, Peter Maydell wrote:
>  On 8 July 2014 18:20, Al Viro <viro@zeniv.linux.org.uk> wrote:
>> > On Tue, Jul 08, 2014 at 05:33:16PM +0100, Peter Maydell wrote:
<snip>
> Again, gprof isn't particulary useful - kernel-side profilers are at least as
> good.  So I suspect that most of the people running into that simply shrug and
> use those instead.  Narrowing it down to -pie didn't take long and I can
> confirm that this is the root cause of that breakage.  Should make debugging
> said toolchain bug a bit easier, if anybody cares to do that...
>
>> > 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.)
>
> Might be a fun project to teach perf that hits in such-and-such page should
> lead to lookup in a table annotating it.  As in "offsets 42..69 should be
> recorded as (<this address> + offset - 42).  Then tcg could generate
> such tables and we'd get information like "that much time is spent in
> the second host insn of instances of that code pattern generated by
> tcg_gen_shr_i64", etc.
>
> No idea if anything of that sort exists - qemu is not the only possible user
> for that; looks like it might be useful for any JIT profiling, so somebody
> could've done that already...

Handily our patch tracker has remembered what I couldn't find ;-)

https://patches.linaro.org/27229/

As I mentioned previously I plan to clean these up over the next week.

-- 
Alex Bennée