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

[Paolo Bonzini]

On 22/02/2018 18:06, John Snow wrote:
> 
> 
> On 02/22/2018 05:57 AM, Kevin Wolf wrote:
>> 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.
>>
> 
> It hasn't been a priority for me.
> 
> Paolo tried to fix ATAPI by adding a BH callback, but that added the
> possibility of a migration halfway through a data transfer IIRC.
> 
> If anyone wants to tackle it, I'll dig up Paolo's patches.

A better possibility is to make it into tail recursion first and then a
while loop.  Maybe introducing some kind of ide_transfer_start_norecurse
that returns "true" if you have a start_transfer callback (so you need
to do another iteration immediately) and "false" if you don't.  I'll
take a look...

Paolo