Dear KVM Developers,

i have a strange problem with segfaults in a guest system
while compiling a linux kernel.

The basic setup (details below) is a ryzen cpu on a ab350 mainboard,
the host running ubuntu linux and guests running also debian based
linux.
Now i am compiling a vanilla linux kernel in the guest.
This will fail with random segfaults as soon as i enable a WLAN (USB
RTL8187L) connection in
the host system.
Apart from this everything is working fine.
As this WLAN is my default internet connection i did try different
virtualization setups
for almost a week until I could track down the problem.


The details:
CPU: Ryzen 7 2700X
same result using a Ryzen 5 1600

Mainboard: GA-AB350M-Gaming 3
Bios: i tested with three versions:  F31, F41d, F50a

host system:
i tested with the following systems as host all x86_64
ubuntu 19.10 kernel 5.3.0-26-generic
ubuntu 19.10 kernel 5.3.0-29-generic
ubuntu 20(development) kernel 5.4.0-14-generic

kvm:
4.0 (ubutu 19.10) and 4.2.0 (ubuntu 19.10 installed from sources)
4.2.0 (ubuntu 20 dev)

guest:
ubuntu 18.10 kernel 4.18.0-10-generic
debian 10 kernel 5.4.0-3-amd64

I use libvirt to launch the guests, so two example XML files are attached,
also the hypervisor info.
I tried a lot of different settings for cpu type and and cpu pinning
without much
difference.

task:
Compile linux kernel on guest using debian linuc sources 5.4.8 or
vanilla 5.5.2

This works fine as long as I do not enable my WLAN internet connection
on the host (USB stick: NetGear, Inc. WG111v2 54 Mbps Wireless [RealTek
RTL8187L])

If I enable the WLAN (in the host) the guest starts producing random
compiler errors like these;

./include/linux/mm.h:2041:1: internal compiler error: Segmentation fault 
static inline spinlock_t *pud_lockptr(struct mm_struct *mm, pud_t *pud)                                                                        
^~~~~~

./include/linux/quota.h:338:1: internal compiler error: in tree_to_uhwi, at tree.h:4278
 };
 ^
./arch/x86/include/asm/pgtable.h:163:1: internal compiler error: Segmentation fault
 {
 ^
elf.c:685:1: internal compiler error: Segmentation fault
 }
 ^

As soon as I disable the USB WLAN connection on the host (the USB stick
still
inserted) I can compile the kernels without any problem.

No valuable messages in the log files of the host and the guest system.

So I tested removing all Network Interfaces from the guest systems,
but this had no effect. Even without any networking a guest fails
compiling as soon as I
start the WLAN on the host system.

I checked for electrical interference by
- using a long USB cable to get the WLAN stick away from the PC ->
compilation fails
- using a laptop operating the USB WLAN stick right beside the PC ->
compilation works fine

I intensely tested the hosts RAM with memtest86+
stressed the host system with compile jobs,
even did memtests on the guests.
But everythings seems rock solid.

Does anyone have a clue, whats going on here,
or how i can debug this situation ?

Thanks,
Torsten