Re: [PATCH] hw/net: move allocation to the heap due to very large stack frame

[David Gibson]

On Sat, Oct 10, 2020 at 08:53:00AM -0700, Elena Afanasova wrote:
> On Sat, 2020-10-10 at 17:07 +1100, David Gibson wrote:
> > On Fri, Oct 09, 2020 at 07:02:56AM -0700, Elena Afanasova wrote:
> > > > From 09905773a00e417d3a37c12350d9e55466fdce8a Mon Sep 17 00:00:00
> > > > 2001
> > > From: Elena Afanasova <eafanasova@gmail.com>
> > > Date: Fri, 9 Oct 2020 06:41:36 -0700
> > > Subject: [PATCH] hw/net: move allocation to the heap due to very
> > > large stack
> > >  frame
> > 
> > Patch looks fine, but some more details of the motivation would be
> > nice.  I wouldn't have thought that the size of a network packet
> > counted as a "very large" stack frame by userspace standards.
> > 
> 
> gcc with wstack-usage warns that stack frame size may be ~65Kbytes

AFAICT, -Wstack-usage takes a parameter giving what size it will
complain about.  What was that value, and what was the rationale for
choosing it?

> 
> > > Signed-off-by: Elena Afanasova <eafanasova@gmail.com>
> > > ---
> > >  hw/net/spapr_llan.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/hw/net/spapr_llan.c b/hw/net/spapr_llan.c
> > > index 2093f1bad0..581320a0e7 100644
> > > --- a/hw/net/spapr_llan.c
> > > +++ b/hw/net/spapr_llan.c
> > > @@ -688,7 +688,8 @@ static target_ulong
> > > h_send_logical_lan(PowerPCCPU *cpu,
> > >      SpaprVioDevice *sdev = spapr_vio_find_by_reg(spapr->vio_bus,
> > > reg);
> > >      SpaprVioVlan *dev = VIO_SPAPR_VLAN_DEVICE(sdev);
> > >      unsigned total_len;
> > > -    uint8_t *lbuf, *p;
> > > +    uint8_t *p;
> > > +    g_autofree uint8_t *lbuf = NULL;
> > >      int i, nbufs;
> > >      int ret;
> > >  
> > > @@ -729,7 +730,7 @@ static target_ulong
> > > h_send_logical_lan(PowerPCCPU *cpu,
> > >          return H_RESOURCE;
> > >      }
> > >  
> > > -    lbuf = alloca(total_len);
> > > +    lbuf = g_malloc(total_len);
> > >      p = lbuf;
> > >      for (i = 0; i < nbufs; i++) {
> > >          ret = spapr_vio_dma_read(sdev, VLAN_BD_ADDR(bufs[i]),
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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