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Re: [Qemu-devel] [PATCH v2 0/6] ARM: AREG0 conversion


From: Lluís Vilanova
Subject: Re: [Qemu-devel] [PATCH v2 0/6] ARM: AREG0 conversion
Date: Mon, 26 Mar 2012 19:59:47 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.94 (gnu/linux)

Blue Swirl writes:

> On Mon, Mar 26, 2012 at 13:05, Paul Brook <address@hidden> wrote:
>>> On 24 March 2012 18:58, Blue Swirl <address@hidden> wrote:
>>> > v2: fix patch 1, tweak patch 2 and rebase to master.
>>> >
>>> > URL     git://repo.or.cz/qemu/blueswirl.git
>>> >        http://repo.or.cz/r/qemu/blueswirl.git
>>> >
>>> > Blue Swirl (6):
>>> >  arm: move neon_tbl to neon_helper.c
>>> >  arm: move saturating arithmetic to helper.c
>>> >  arm: move other arithmetic to helper.c
>>> >  arm: move cpsr and banked register access to helper.c
>>> >  arm: move exception and wfi helpers to helper.c
>>> >  arm: move load and store helpers, switch to AREG0 free mode
>>> 
>>> The patches themselves look OK, but do we really want to take
>>> a 5% performance hit for this cleanup?
>> 
>> I have a similar concern.  I'd like to at least have some idea where this
>> slowdown is coming from.

> At least stack protector is protecting more code than before (for
> example TLB miss handler), but could overhead from that amount to 5%?

Then you can try comparing both builds with a modified configure that does not
add the "-fstack-protector-all" option.

If you want to fine-tune it, you can add
"__attribute__((optimize("no-stack-protector")))" to those functions or just
add:

    #pragma GCC push_options
    #pragma GCC optimize ("no-stack-protector")

at the beginning of the "softmmu_template.h", and:

    #pragma GCC pop_options

at the end of it.

Or even better, use it for the whole "target-*/*helper.c" file, as there should
be no user-induced overflow in helpers (unless the instr decoding code in
"translate.c" is exploitable).


Thanks,
  Lluis

-- 
 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom
 Tollbooth



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