Re: [Qemu-devel] [Qemu-block] Limiting coroutine stack usage

[Peter Lieven]

Am 22.02.2018 um 13:03 schrieb Daniel P. Berrangé:
> On Thu, Feb 22, 2018 at 01:02:05PM +0100, Peter Lieven wrote:
>> Am 22.02.2018 um 13:00 schrieb Daniel P. Berrangé:
>>> On Thu, Feb 22, 2018 at 12:51:58PM +0100, Peter Lieven wrote:
>>>> Am 22.02.2018 um 12:40 schrieb Daniel P. Berrangé:
>>>>> On Thu, Feb 22, 2018 at 12:32:04PM +0100, Kevin Wolf wrote:
>>>>>> Am 22.02.2018 um 12:01 hat Peter Lieven geschrieben:
>>>>>>> Am 22.02.2018 um 11:57 schrieb Kevin Wolf:
>>>>>>>> Am 20.02.2018 um 22:54 hat Paolo Bonzini geschrieben:
>>>>>>>>> On 20/02/2018 18:04, Peter Lieven wrote:
>>>>>>>>>> Hi,
>>>>>>>>>>
>>>>>>>>>> I remember we discussed a long time ago to limit the stack usage of 
>>>>>>>>>> all
>>>>>>>>>> functions that are executed in a coroutine
>>>>>>>>>> context to a very low value to be able to safely limit the coroutine
>>>>>>>>>> stack size as well.
>>>>>>>>> IIRC the only issue was that hw/ide/atapi.c has mutual recursion 
>>>>>>>>> between
>>>>>>>>> ide_atapi_cmd_reply_end -> ide_transfer_start -> ahci_start_transfer 
>>>>>>>>> ->
>>>>>>>>> ide_atapi_cmd_reply_end.
>>>>>>>>>
>>>>>>>>> But perhaps it's not an issue, somebody needs to audit the code.
>>>>>>>> I think John intended to get rid of the recursion sometime, but I doubt
>>>>>>>> he has had the time so far.
>>>>>>> Apart from this is is possible to define special cflags in the
>>>>>>> Makefile.objs just for a subdirectory? I have patches ready to make
>>>>>>> the block layer files and other coroutine users compile with
>>>>>>> -Wstack-size=2048. But I do not want to specify each file separately.
>>>>>> Our Makefiles have lines like this:
>>>>>>
>>>>>>     iscsi.o-cflags     := $(LIBISCSI_CFLAGS)
>>>>>>
>>>>>> I don't think there is a direct mechanism to apply cflags to a whole
>>>>>> directory or just to block-obj-y/block-obj-m, but just looping over them
>>>>>> could work. I'm not a Makefile expert at all, but after some toying with
>>>>>> a simple example, something like this might work:
>>>>>>
>>>>>>     $(foreach x,$(block-obj-y),$(eval $x-cflags += -Wstack-size=2048))
>>>>> You'll need it for anything block layer depends on too - so that's much
>>>>> of util/, crypto/ and io/ directories at least.
>>>>>
>>>>> So perhaps it would be shorter if we do the opposite - set 
>>>>> -Wstack-size=2048
>>>>> globally for everything in QEMU, and then override -Wstack-size=$BIGGER
>>>>> for the (hopefully) few sources that have a larger stack need ?
>>>> I tried that already. 2048 is a strong limit for many functions.
>>>> It breaks already as soon as some buffer has a size of PATH_MAX, but
>>>> thats handleable. But there are some structs around that are very large.
>>> There are surprisingly few "char [PATH_MAX]" variables left in QEMU - we
>>> should have a final push to eliminate them regardless.
>>>
>>>> Generally, it would be a good idea to have a global limit, of course.
>>> We could at least put a limit on that matches the current worst case to
>>> prevent it getting worse than it already is.
>> That would be a good idea, yes.
>>
>> How would you handle the override for a smaller -Wstack-usage ?
> If you have multiple -Wstack-size=$XXX  flags to GCC, I expect the last
> one wins. So just need to double check that the per-object file CFLAGS
> occur after the global CFLAS in the compiler args

I will check that, thanks.

When I am at it, what would be the proper replacement for char[PATH_MAX] ?

Peter