Re: [PATCH net-next v3 2/9] xen-netback: Change TX path from grant copy to mapping

[Wei Liu <wei.liu2@citrix.com>]

On Fri, Jan 10, 2014 at 11:45:34AM +0000, Wei Liu wrote:
> On Fri, Jan 10, 2014 at 11:35:08AM +0000, Zoltan Kiss wrote:
> [...]
> > 
> > >>@@ -920,6 +852,18 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
> > >>  	err = gop->status;
> > >>  	if (unlikely(err))
> > >>  		xenvif_idx_release(vif, pending_idx, XEN_NETIF_RSP_ERROR);
> > >>+	else {
> > >>+		if (vif->grant_tx_handle[pending_idx] !=
> > >>+			NETBACK_INVALID_HANDLE) {
> > >>+			netdev_err(vif->dev,
> > >>+				"Stale mapped handle! pending_idx %x handle %x\n",
> > >>+				pending_idx, vif->grant_tx_handle[pending_idx]);
> > >>+			BUG();
> > >>+		}
> > >>+		set_phys_to_machine(idx_to_pfn(vif, pending_idx),
> > >>+			FOREIGN_FRAME(gop->dev_bus_addr >> PAGE_SHIFT));
> > >
> > >What happens when you don't have this?
> > Your frags will be filled with garbage. I don't understand exactly
> > what this function does, someone might want to enlighten us? I've
> > took it's usage from classic kernel.
> > Also, it might be worthwhile to check the return value and BUG if
> > it's false, but I don't know what exactly that return value means.
> > 
> 
> This is actually part of gnttab_map_refs. As you're using hypercall
> directly this becomes very fragile.
> 

To make it clear, set_phys_to_machine is done within m2p_add_override.

Wei.