virtio_net: Set rx/tx busdma alignment to 1 byte

[sarah.walker2_arm.com]

Summary

Previous alignment is unnecessary, and caused locking issues on amd64.

Sponsored by: Arm Ltd

[markj]

Please be sure to add a Fixes: tag to the commit log message.

sys/dev/virtio/network/if_vtnet.c

487

Just write 1?

[sarah.walker2_arm.com]

Address review comments

[zlei]

How can alignment lead locking issues ? Not an expert on this, just out of curious.

[sarah.walker2_arm.com]

In D56691#1298952, @zlei wrote:

How can alignment lead locking issues ? Not an expert on this, just out of curious.

The standard behaviour for busdma_bounce implementations is that bus_dmamap with alignment > 1 is marked as COULD_BOUNCE. Among other things this can cause a bounce zone to be allocated at bus_dmamap creation time. This causes further allocations without M_NOWAIT which triggers the locking issue.

[zlei]

In D56691#1298953, @sarah.walker2_arm.com wrote:

In D56691#1298952, @zlei wrote:

How can alignment lead locking issues ? Not an expert on this, just out of curious.

The standard behaviour for busdma_bounce implementations is that bus_dmamap with alignment > 1 is marked as COULD_BOUNCE. Among other things this can cause a bounce zone to be allocated at bus_dmamap creation time. This causes further allocations without M_NOWAIT which triggers the locking issue.

WoW !

[andrew]

This can be closed? It's been merged into th re-opened D55492