From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mukesh Rathor <mukesh.rathor@oracle.com>
Subject: Re: [BUGFIX][PATCH 4/4] XEN_DOMCTL_gdbsx_guestmemio:
 always do the copyback.
Date: Mon, 6 Jan 2014 17:53:49 -0800
Message-ID: <20140106175349.6cbd190b@mantra.us.oracle.com>
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To: Don Slutz <dslutz@verizon.com>
Cc: Keir Fraser <keir@xen.org>, Ian Campbell <ian.campbell@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, George Dunlap <george.dunlap@eu.citrix.com>, Ian Jackson <ian.jackson@eu.citrix.com>, xen-devel@lists.xen.org, Jan Beulich <jbeulich@suse.com>
List-Id: xen-devel@lists.xenproject.org

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