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Differential D56691

virtio_net: Set rx/tx busdma alignment to 1 byte
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Authored by sarah.walker2_arm.com on Apr 28 2026, 1:43 PM.
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andrew
imp
zlei

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markj
Summary

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

Sponsored by: Arm Ltd

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rG FreeBSD src repository
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Buildable 72634
Build 69517: arc lint + arc unit

Event Timeline

sarah.walker2_arm.com created this revision.Apr 28 2026, 1:43 PM
Herald added a subscriber: imp. · View Herald TranscriptApr 28 2026, 1:43 PM
sarah.walker2_arm.com requested review of this revision.Apr 28 2026, 1:43 PM
Harbormaster completed remote builds in B72603: Diff 176716.Apr 28 2026, 1:43 PM
andrew added a reviewer: markj.Apr 28 2026, 1:44 PM
markj accepted this revision.Apr 28 2026, 9:40 PM

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

sys/dev/virtio/network/if_vtnet.c
487

Just write 1?

This revision is now accepted and ready to land.Apr 28 2026, 9:40 PM
sarah.walker2_arm.com updated this revision to Diff 176773.Apr 29 2026, 7:26 AM

Address review comments

This revision now requires review to proceed.Apr 29 2026, 7:26 AM
Harbormaster completed remote builds in B72634: Diff 176773.Apr 29 2026, 7:26 AM
markj accepted this revision.Apr 29 2026, 1:18 PM
This revision is now accepted and ready to land.Apr 29 2026, 1:18 PM
zlei added a subscriber: zlei.Apr 29 2026, 1:19 PM

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

sarah.walker2_arm.com added a comment.Apr 29 2026, 1:29 PM
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 added a comment.Apr 29 2026, 3:48 PM
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 mentioned this in D55492: virtio_net: Use bus_dma for rxq/txq buffers.May 12 2026, 9:19 AM
andrew added a subscriber: andrew.

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

sarah.walker2_arm.com abandoned this revision.May 12 2026, 3:17 PM

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sys/
dev/
virtio/
network/
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Diff 176773

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sys/dev/virtio/network/if_vtnet.c

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