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Re: [Qemu-devel] [PATCH v2] tcg: Remove stack protection from helper fun

From: Blue Swirl
Subject: Re: [Qemu-devel] [PATCH v2] tcg: Remove stack protection from helper functions
Date: Mon, 26 Sep 2011 18:40:54 +0000

On Mon, Sep 26, 2011 at 6:25 PM, Jan Kiszka <address@hidden> wrote:
> On 2011-09-26 20:20, Blue Swirl wrote:
>> On Mon, Sep 26, 2011 at 5:33 PM, Jan Kiszka <address@hidden> wrote:
>>> On 2011-09-26 19:22, Blue Swirl wrote:
>>>> On Mon, Sep 26, 2011 at 11:56 AM, Peter Maydell
>>>> <address@hidden> wrote:
>>>>> On 26 September 2011 12:43, Jan Kiszka <address@hidden> wrote:
>>>>>> On 2011-09-26 13:33, Peter Maydell wrote:
>>>>>>> On 26 September 2011 11:51, Jan Kiszka <address@hidden> wrote:
>>>>>>>> This increases the overhead of frequently executed helpers. We need to
>>>>>>>> move rule past QEMU_CFLAGS assignment to ensure that the required 
>>>>>>>> simple
>>>>>>>> assignment picks up all bits. The signal workaround is moved just for
>>>>>>>> the sake of consistency.
>>>>>>>> +# NOTE: Must be after the last QEMU_CFLAGS assignment
>>>>>>>> +op_helper.o user-exec.o: QEMU_CFLAGS := $(subst 
>>>>>>>> -fstack-protector-all,,$(QEMU_CFLAGS)) $(HELPER_CFLAGS)
>>>>>>> Why also user-exec.o ?
>>>>>> That's a good question. It doesn't look like it's deserving this.
>>>>>>> Why not the other source files with helpers in?
>>>>>> Name them and I add them.
>>>>> target-*/*helper.c ?
>>>>> But mostly I think what I'm trying to say is that this is making
>>>>> a tradeoff between safety and speed, so it ought to come with a
>>>>> rationale for why it is OK to remove the safety checks for these
>>>>> files. Given that rationale you can identify other files that are
>>>>> also safe/worthwhile to flip the flag for.
>>>> I wouldn't remove -fstack-protector-all by default. Especially op code
>>>> interfaces with the guest.
>>> I'd love to have some function attribute for this, because a stack
>>> protector for rather simple arithmetic operations or something like
>>> helper_cli/sti are pointlessly burned cycles.
>> In order to avoid burning the cycles, there is a certain kernel module
>> which gives almost native performance.
> Well, even in 2011 there are cases remaining where VT-x/AMV-V is not
> available to your favorite hypervisor.
>>> Maybe we can introduce op_helper_simple.c.
>>>> For max performance version, I'd check if -fomit-frame-pointer and -O3
>>>> makes sense. See also this article:
>>>> https://www.debian-administration.org/article/672/Optimizing_code_via_compiler_flags
>>> We already run without frame pointers, -O3 might be worth exploring in
>>> addition. Still, that won't take the protector overhead away.
>> It would be interesting to have some benchmarks. I'd expect that most
>> of the run time is spent within generated code, the next largest item
>> should be the translator and any helpers should be marginal.
> At least we've a rather static, long-running guest, not much is
> recompiled once the system has settled.

Optimizing the translator or improving TCG optimizer could also
improve performance.

By the way, -fomit-frame-pointer is not enabled globally for some
reason, it should be the default for non-debug builds.

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