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

[Jan Kiszka]

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.

Jan

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