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

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>
Cc: Peter Lieven <pl@kamp.de>, Paolo Bonzini <pbonzini@redhat.com>,
	jsnow@redhat.com, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	qemu block <qemu-block@nongnu.org>
Subject: Re: [Qemu-devel] [Qemu-block] Limiting coroutine stack usage
Date: Thu, 22 Feb 2018 11:40:53 +0000	[thread overview]
Message-ID: <20180222114053.GE9323@redhat.com> (raw)
In-Reply-To: <20180222113204.GC4147@localhost.localdomain>

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 ?

Regards,
Daniel
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