Re: [PATCH] xen/netfront: Fix TX response spurious interrupts

[Juergen Gross <jgross@xxxxxxxx>]

On 10.07.25 18:11, Anthoine Bourgeois wrote:
> We found at Vates that there are lot of spurious interrupts when
> benchmarking the PV drivers of Xen. This issue appeared with a patch
> that addresses security issue XSA-391 (see Fixes below). 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 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.
>
> 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
>
> Fixes: b27d47950e48 ("xen/netfront: harden netfront against event channel 
> storms")
> Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@xxxxxxxxxx>
Please resend this patch with the relevant maintainers added in the
recipients list.

You can add my Reviewed-by: tag, of course.


Juergen

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