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

["Anthoine Bourgeois" <anthoine.bourgeois@vates.tech>]

On Fri, Jul 11, 2025 at 05:33:43PM +0200, Juergen Gross wrote:
>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@vates.tech>
>
>Please resend this patch with the relevant maintainers added in the
>recipients list.

Ok, I will resend the patch tomorrow.
>
>You can add my Reviewed-by: tag, of course.

Thanks!

Anthoine






Anthoine Bourgeois | Vates XCP-ng Developer

XCP-ng & Xen Orchestra - Vates solutions

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