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Re: [PATCH] xen-netback: correct success/error reporting for the SKB-with-fraglist case

On 25/02/2021 07:33, Jan Beulich wrote:
On 24.02.2021 17:39, Paul Durrant wrote:
On 23/02/2021 16:29, Jan Beulich wrote:
When re-entering the main loop of xenvif_tx_check_gop() a 2nd time, the
special considerations for the head of the SKB no longer apply. Don't
mistakenly report ERROR to the frontend for the first entry in the list,
even if - from all I can tell - this shouldn't matter much as the overall
transmit will need to be considered failed anyway.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -499,7 +499,7 @@ check_frags:
                                 * the header's copy failed, and they are
                                 * sharing a slot, send an error
-                               if (i == 0 && sharedslot)
+                               if (i == 0 && !first_shinfo && sharedslot)
                                        xenvif_idx_release(queue, pending_idx,

I think this will DTRT, but to my mind it would make more sense to clear
'sharedslot' before the 'goto check_frags' at the bottom of the function.

That was my initial idea as well, but
- I think it is for a reason that the variable is "const".
- There is another use of it which would then instead need further
   amending (and which I believe is at least part of the reason for
   the variable to be "const").

Oh, yes. But now that I look again, don't you want:

if (i == 0 && first_shinfo && sharedslot)

? (i.e no '!')

The comment states that the error should be indicated when the first frag contains the header in the case that the map succeeded but the prior copy from the same ref failed. This can only possibly be the case if this is the 'first_shinfo' (which is why I still think it is safe to unconst 'sharedslot' and clear it).





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