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

[Mukesh Rathor <mukesh.rathor@oracle.com>]

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);
        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