Re: [BUGFIX][PATCH 4/4] XEN_DOMCTL_gdbsx_guestmemio: always do the copyback.

[Ian Campbell <Ian.Campbell@citrix.com>]

On Mon, 2014-01-06 at 17:53 -0800, Mukesh Rathor wrote:
> On Sat,  4 Jan 2014 12:52:16 -0500
> Don Slutz <dslutz@verizon.com> wrote:
> 
> > The gdbsx code expects that domctl->u.gdbsx_guest_memio.remain is
> > returned.
> > 
> > Without this gdb does not report an error.
> > 
> > With this patch and using a 1G hvm domU:
> > 
> > (gdb) x/1xh 0x6ae9168b
> > 0x6ae9168b:     Cannot access memory at address 0x6ae9168b
> > 
> > Signed-off-by: Don Slutz <dslutz@verizon.com>
> > ---
> >  xen/arch/x86/domctl.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
> > index ef6c140..4aa751f 100644
> > --- a/xen/arch/x86/domctl.c
> > +++ b/xen/arch/x86/domctl.c
> > @@ -997,8 +997,7 @@ long arch_do_domctl(
> >              domctl->u.gdbsx_guest_memio.len;
> >  
> >          ret = gdbsx_guest_mem_io(domctl->domain,
> > &domctl->u.gdbsx_guest_memio);
> > -        if ( !ret )
> > -           copyback = 1;
> > +        copyback = 1;
> >      }
> >      break;
> >  
> 
> Ooopsy... my thought was that an application should not even look at
> remain if the hcall/syscall failed, but forgot when writing the
> gdbsx itself :). Think of it this way, if the call didn't even make it to 
> xen, and some reason the ioctl returned non-zero rc, then remain would 
> still be zero. So I think we should fix gdbsx instead of here:
> 
> xg_write_mem():
>     if ((rc=_domctl_hcall(XEN_DOMCTL_gdbsx_guestmemio, frombuf, buflen)))
>     {
>         XGERR("ERROR: failed to write %d bytes. errno:%d rc:%d\n",
>                iop->remain, errno, rc);

Isn't this still using iop->remain on failure which is what you say
shouldn't be done?

>         return iop->len;
>     }    
> 
> Similarly in xg_read_mem().
> 
> Hope that makes sense. Don't mean to create work for you for my mistake,
> so if you don't have time, I can submit a patch for this too.
> 
> thanks
> Mukesh