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Re: [Qemu-discuss] xHCI controller/USB passthrough device - event ring f

From: Gerd Hoffmann
Subject: Re: [Qemu-discuss] xHCI controller/USB passthrough device - event ring full error on high packet rate
Date: Thu, 1 Feb 2018 11:38:43 +0100
User-agent: NeoMutt/20171215

On Thu, Dec 07, 2017 at 05:52:25PM +1000, Samuel Brian wrote:
> Hi all,

Sorry, for the long delay, was offline for a few weeks due to a broken
finger and now I'm going back in my inbox looking for unanswered mails.

>   xhci_hcd 0000:00:05.0: ERROR unknown event type 37

> Qemu gives me this error message before it sends that event type 37

Seems linux simply hasn't implemented that ...

> I think my problem is sort of mentioned in
> http://lists.nongnu.org/archive/html/qemu-devel/2017-02/msg00598.html
> > It does suspious ERDP updates now and
> > then (not every time, seems we must hit a race window for this to
> > happen), which in turn makes the qemu xhci emulation think the event
> > ring is full.  Things go south from here ...
> I suppose I'm just really good at hitting that race window

I don't think so.  If you are running latest qemu (2.11, I think 2.10 is
new enough too) this should be fixed in qemu.  If you are running
something older try upgrading.

> -- I can udpblast
> at 130,000 network packets per second over the modem when not running in a
> VM.

Probably the event ring is _really_ full.  The linux kernel in the guest
probably can't unqueue the events fast enough due to virtualization
overhead and/or the vcpu not being scheduled for a while.

> For reference, I'm using Qemu v2.10.1 and libusb 1.0.21 and running the VM
> on a Linux 4.14 host with:

Who drives the modem in the guest?  libusb too?  Or some kernel driver?

I would try to reduce the number of events, by asking xhci for an
interrupt on every 10th transfer, so the irq rate in the guest goes down
from 130000 to 13000.  Not sure there is an easy way to do so though.


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