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

[Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>]

On 10/01/14 11:45, 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.
>
> So the right thing to do is to fix gnttab_map_refs.
I agree, as I mentioned in other email in this thread, I think that 
should be the topic of an another patchseries. In the meantime, I will 
use gnttab_batch_map instead of the direct hypercall, it handles the 
GNTST_eagain scenario, and I will use set_phys_to_machine the same way 
as m2p_override does:
if (unlikely(!set_phys_to_machine(idx_to_pfn(vif, pending_idx), 
FOREIGN_FRAME(gop->dev_bus_addr >> PAGE_SHIFT))
                        BUG();

Zoli

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