Re: Testing the virtio-vhost-user QEMU patch

[Alyssa Ross <hi@alyssa.is>]

Stefan Hajnoczi <stefanha@redhat.com> writes:

> On Fri, Jul 24, 2020 at 10:58:45AM +0000, Alyssa Ross wrote:
>> Alyssa Ross <hi@alyssa.is> writes:
>> 
>> > 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.
>> >
>> > Small update here: I tried on another computer, and it worked.  Made
>> > sure that it was exactly the same QEMU binary, command line, and VM
>> > disk/initrd/kernel, so I think I can fairly confidently say the panic
>> > depends on what hardware QEMU is running on.  I set -cpu value to the
>> > same on both as well (SandyBridge).
>> >
>> > I also discovered that it works on my primary computer (the one it
>> > panicked on before) with KVM disabled.
>> >
>> > Note that I've only got so far as finding that it boots on the other
>> > machine -- I haven't verified yet that it actually works.
>> >
>> > Bad host CPU:  Intel(R) Core(TM) i5-2520M CPU @ 2.50GHz
>> > Good host CPU: AMD EPYC 7401P 24-Core Processor
>> >
>> > May I ask what host CPUs other people have tested this on?  Having more
>> > data would probably be useful.  Could it be an AMD vs. Intel thing?
>> 
>> I think I've figured it out!
>> 
>> Sandy Bridge and Ivy Bridge hosts encounter this panic because the
>> "additional resources" bar size is too big, at 1 << 36.  If I change
>> this to 1 << 35, no more kernel panic.
>> 
>> Skylake and later are fine with 1 << 36.  In between Ivy Bridge and
>> Skylake were Haswell and Broadwell, but I couldn't find anybody who was
>> able to help me test on either of those, so I don't know what they do.
>> 
>> Perhaps related, the hosts that produce panics all seem to have a
>> physical address size of 36 bits, while the hosts that work have larger
>> physical address sizes, as reported by lscpu.
>
> I have run it successfully on Broadwell but never tried 64GB or larger
> shared memory resources.

To clarify, I haven't been using big shared memory resources either --
this has all been about getting the backend VM to start at all.  The
panic happens at boot, and the 1 << 36 BAR allocation comes from here,
during realization:
https://github.com/ndragazis/qemu/blob/f9ab08c0c8/hw/virtio/virtio-vhost-user-pci.c#L291