5.15-stable review patch. If anyone has any objections, please let me know.
------------------
From: Anthoine Bourgeois anthoine.bourgeois@vates.tech
[ Upstream commit 114a2de6fa86d99ed9546cc9113a3cad58beef79 ]
We found at Vates that there are lot of spurious interrupts when benchmarking the xen-net PV driver frontend. This issue appeared with a patch that addresses security issue XSA-391 (b27d47950e48 "xen/netfront: harden netfront against event channel storms"). On an iperf benchmark, spurious interrupts can represent up to 50% of the interrupts.
Spurious interrupts are interrupts that are rised for nothing, there is no work to do. This appends because the function that handles the interrupts ("xennet_tx_buf_gc") is also called at the end of the request path to garbage collect the responses received during the transmission load.
The request path is doing the work that the interrupt handler should have done otherwise. This is particurary true when there is more than one vcpu and get worse linearly with the number of vcpu/queue.
Moreover, this problem is amplifyed by the penalty imposed by a spurious interrupt. When an interrupt is found spurious the interrupt chip will delay the EOI to slowdown the backend. This delay will allow more responses to be handled by the request path and then there will be more chance the next interrupt will not find any work to do, creating a new spurious interrupt.
This causes performance issue. The solution here is to remove the calls from the request path and let the interrupt handler do the processing of the responses. This approch removes most of the spurious interrupts (<0.05%) and also has the benefit of freeing up cycles in the request path, allowing it to process more work, which improves performance compared to masking the spurious interrupt one way or another.
This optimization changes a part of the code that is present since the net frontend driver was upstreamed. There is no similar pattern in the other xen PV drivers. Since the first commit of xen-netfront is a blob that doesn't explain all the design choices I can only guess why this specific mecanism was here. This could have been introduce to compensate a slow backend at the time (maybe the backend was fixed or optimize later) or a small queue. In 18 years, both frontend and backend gain lot of features and optimizations that could have obsolete the feature of reaping completions from the TX path.
Some vif throughput performance figures from a 8 vCPUs, 4GB of RAM HVM guest(s):
Without this patch on the : vm -> dom0: 4.5Gb/s vm -> vm: 7.0Gb/s
Without XSA-391 patch (revert of b27d47950e48): vm -> dom0: 8.3Gb/s vm -> vm: 8.7Gb/s
With XSA-391 and this patch: vm -> dom0: 11.5Gb/s vm -> vm: 12.6Gb/s
v2: - add revewed and tested by tags - resend with the maintainers in the recipients list
v3: - remove Fixes tag but keep the commit ref in the explanation - add a paragraph on why this code was here
Signed-off-by: Anthoine Bourgeois anthoine.bourgeois@vates.tech Reviewed-by: Juergen Gross jgross@suse.com Tested-by: Elliott Mitchell ehem+xen@m5p.com Signed-off-by: Juergen Gross jgross@suse.com Message-ID: 20250721093316.23560-1-anthoine.bourgeois@vates.tech Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/net/xen-netfront.c | 5 ----- 1 file changed, 5 deletions(-)
diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c index 0add51f1b7e4..4b19d54fa2e2 100644 --- a/drivers/net/xen-netfront.c +++ b/drivers/net/xen-netfront.c @@ -639,8 +639,6 @@ static int xennet_xdp_xmit_one(struct net_device *dev, tx_stats->packets++; u64_stats_update_end(&tx_stats->syncp);
- xennet_tx_buf_gc(queue); - return 0; }
@@ -850,9 +848,6 @@ static netdev_tx_t xennet_start_xmit(struct sk_buff *skb, struct net_device *dev tx_stats->packets++; u64_stats_update_end(&tx_stats->syncp);
- /* Note: It is not safe to access skb after xennet_tx_buf_gc()! */ - xennet_tx_buf_gc(queue); - if (!netfront_tx_slot_available(queue)) netif_tx_stop_queue(netdev_get_tx_queue(dev, queue->id));