Re: [Qemu-devel] [PATCH v5 24/24] slirp: fix ipv6 timers

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Samuel Thibault [mailto:samuel.thibault@gnu.org]
> Pavel Dovgalyuk, le jeu. 26 juil. 2018 10:08:29 +0300, a ecrit:
> > virtual clock should be used by the virtual devices.
> > slirp module is not the virtual device. Therefore processed packets
> > become visible to the guest after passing to the virtual network card.
> > Before that it can create any timers that should not change the state of the guest.
> 
> I'm not sure I understand that part correctly. slirp is not a "device"
> strictly speaking, but it has a whole foot in the virtual world. All
> TCP/UDP/ARP/RA timings are related to the guest timing, so

I don't know all details of slirp, so let me ask:
if the virtual timer runs very slowly (when it configured this way with icount option),
should the timings relate this speed? Or the timers are related to the network devices
(e.g., servers in the outer world)?

> > > > this service is not related to the guest state.
> 
> seems incorrect. At the moment the ip6_icmp timer's current value is not
> saved in the guest state, but in principle it should, so that the guest
> does see the RAs at a regular rate. In practice we don't care because
> the timing is randomized anyway.

Isn't this just a side effect?
I mean that slirp may be replaced by, say, tap, and the guest should not notice
the difference.

> > intended to be used for the internal QEMU purposes, but stops when VM
> > is stopped.
> 
> I again don't understand this. The ip6_icmp timing is not for internal
> QEMU purpose, its whole point is how often RAs are sent to the guest.
> 
> slirp's guest part is not a device as directly seen by guest I/O, but
> it's a router device as seen through guest packets. Think of it like a
> USB device, which is seen by the guest through USB packets.

Record/replay implementation creates a line between the guest state and
the outer world. Everything crossing this line is saved in the log replayed.
In case of network, this line is implemented with the network filter.
It takes packets from slirp(or anything) and passes(or not) them to the guest nic.
When replaying, the saved packets are injected into the filter directly.
Slirp is the part of the outer world, so it shouldn't affect the guest state directly.

Pavel Dovgalyuk