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

["Daniel P. Berrangé" <berrange@redhat.com>]

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.

Regards,
Daniel
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