Re: Testing the virtio-vhost-user QEMU patch

[Alyssa Ross <hi@alyssa.is>]

Stefan Hajnoczi <stefanha@redhat.com> writes:

> On Tue, Jul 21, 2020 at 07:14:38AM +0000, Alyssa Ross wrote:
>> Hi -- I hope it's okay me reaching out like this.
>> 
>> I've been trying to test out the virtio-vhost-user implementation that's
>> been posted to this list a couple of times, but have been unable to get
>> it to boot a kernel following the steps listed either on
>> <https://wiki.qemu.org/Features/VirtioVhostUser> or
>> <https://ndragazis.github.io/dpdk-vhost-vvu-demo.html>.
>> 
>> Specifically, the kernel appears to be unable to write to the
>> virtio-vhost-user device's PCI registers.  I've included the full panic
>> output from the kernel at the end of this message.  The panic is
>> reproducible with two different kernels I tried (with different configs
>> and versions).  I tried both versions of the virtio-vhost-user I was
>> able to find[1][2], and both exhibited the same behaviour.
>> 
>> Is this a known issue?  Am I doing something wrong?
>
> Hi,
> Unfortunately I'm not sure what the issue is. This is an early
> virtio-pci register access before a driver for any specific device type
> (net, blk, vhost-user, etc) comes into play.
>
> Did you test the git trees linked below or did you rebase the commits
> on top of your own QEMU tree?

I tested the git trees.  For your one I had to make a slight
modification to delete the memfd syscall wrapper in util/memfd.c, since
it conflicted with the one that is now provided by Glibc.  Nikos's tree
I used totally unmodified.

> Is your guest kernel a stock kernel.org/distro kernel or has it been
> modified (especially with security patches)?

I tried a slightly modified Chromium OS kernel (5.4.23), and a stock
Ubuntu 18.10 kernel (4.15.0).  I think the most "normal" setup I tried
was building QEMU on Fedora 32, and then attempting to boot a freshly
installed Ubuntu Server 18.10 VM with

    -chardev socket,id=chardev0,path=vhost-user.sock,server,nowait \
    -device virtio-vhost-user-pci,chardev=chardev0

(The crash was reproducible with the full QEMU command lines in the
write-ups, but these seemed to be the load-bearing bits.)

> If no one else knows what is wrong here then it will be necessary to
> check the Intel manuals to figure out the exact meaning of
> "error_code(0x000b) - reserved bit violation" and why Linux triggers it
> with "PGD 3b128067 P4D 3b128067 PUD 3b129067 PMD 3b12a067 PTE
> 8000002000000073".

Thanks for your insight.  Now I at least have a place to start if nobody
else knows what's up. :)