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Re: [Qemu-devel] [Qemu-ppc] [PATCH] spapr: Implement bug in spapr-vty de


From: Greg Kurz
Subject: Re: [Qemu-devel] [Qemu-ppc] [PATCH] spapr: Implement bug in spapr-vty device to be compatible with PowerVM
Date: Tue, 21 Nov 2017 10:45:23 +0100

On Tue, 21 Nov 2017 08:27:51 +0100
Thomas Huth <address@hidden> wrote:

> On 20.11.2017 08:14, David Gibson wrote:
> > The spapr-vty device implements the PAPR defined virtual console,
> > which is also implemented by IBM's proprietary PowerVM hypervisor.
> > 
> > PowerVM's implementation has a bug where it inserts an extra \0 after
> > every \r going to the guest.  Because of that Linux's guest side
> > driver has a workaround which strips \0 characters that appear
> > immediately after a \r.  
> 
> Ouch.
> 
> I wonder whether it would make more sense to change the guest side
> driver to apply the workaround only if it's really running under
> PowerVM...? E.g. what if the PowerVM bug ever gets fixed one day?
> 

The bug has been around forever (already there in initial linux git
commit 1da177e4c3f4), so I'm not sure PowerVM will ever fix it. It
is legacy API now :) 

Also I'm not sure it is worth the pain to introduce extra complexity
in the existing linux driver... I guess we'd rather encourage people
to switch to virtio serial instead.

Anyway,

Reviewed-by: Greg Kurz <address@hidden>

> > That means that when running under qemu, sending a binary stream from
> > host to guest via spapr-vty which happens to include a \r\0 sequence
> > will get corrupted by that workaround.
> > 
> > To deal with that, this patch duplicates PowerVM's bug, inserting an
> > extra \0 after each \r.  Ugly, but the best option available.
> > 
> > Signed-off-by: David Gibson <address@hidden>
> > ---
> >  hw/char/spapr_vty.c | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> > 
> > diff --git a/hw/char/spapr_vty.c b/hw/char/spapr_vty.c
> > index 0fa416ca6b..a95e5e91a7 100644
> > --- a/hw/char/spapr_vty.c
> > +++ b/hw/char/spapr_vty.c
> > @@ -58,6 +58,24 @@ static int vty_getchars(VIOsPAPRDevice *sdev, uint8_t 
> > *buf, int max)
> >  
> >      while ((n < max) && (dev->out != dev->in)) {
> >          buf[n++] = dev->buf[dev->out++ % VTERM_BUFSIZE];
> > +
> > +        /* PowerVM's vty implementation has a bug where it inserts a
> > +         * \0 after every \r going to the guest.  Existing guests have
> > +         * a workaround for this which removes every \0 immediately
> > +         * following a \r, so here we make ourselves bug-for-bug
> > +         * compatible, so that the guest won't drop a real \0-after-\r
> > +         * that happens to occur in a binary stream. */
> > +        if (buf[n-1] == '\r') {
> > +            if (n < max) {
> > +                buf[n++] = '\0';
> > +            } else {
> > +                /* No room for the extra \0, roll back and try again
> > +                 * next time */
> > +                dev->out--;
> > +                n--;
> > +                break;
> > +            }
> > +        }
> >      }
> >  
> >      qemu_chr_fe_accept_input(&dev->chardev);
> >   
> 
> Code looks fine to me, so:
> 
> Reviewed-by: Thomas Huth <address@hidden>
> 




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