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[Xen-devel] bogus wrap check in xen-netback

With commit 48856286b64e ("xen/netback: shutdown the ring if it contains 
garbage.") a new check was added to xen-netback, which triggers for me.

Since there are bugs in ring buffer handling I tried to trigger them earlier by 
changing RING_IDX from u32 to u16. Now I found another one, and I wonder if the 
error below could potentially also hit with u32:

[  624.186492] br0: port 3(vif2.0) entered forwarding state
[  624.186522] br0: port 3(vif2.0) entered forwarding state
[  680.865398] vif vif-1-0 vif1.0: Impossible number of requests. req_prod 0, 
req_cons 65400, size 256
[  680.865402] vif vif-1-0 vif1.0: fatal error; disabling device
[  680.865495] br0: port 2(vif1.0) entered disabled state
[  689.433849] vif vif-2-0 vif2.0: Impossible number of requests. req_prod 0, 
req_cons 65527, size 256
[  689.433857] vif vif-2-0 vif2.0: fatal error; disabling device
[  689.433945] br0: port 3(vif2.0) entered disabled state
[  690.930512] pktgen: Packet Generator for packet performance testing. 
Version: 2.75

What exactly is that check in xenvif_tx_build_gops trying to achieve? 
Subtracting a non-zero value from zero will always create something larger than 


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