5.10.189 and 5.10.190 breaks nested virtualization

5.10.189 and 5.10.190 breaks nested virtualization

[Blair Strater]

Per the request at https://lwn.net/Articles/940798/, I'm reaching out to let you know that this patch breaks nested virtualization on AMD processors.

I've tested 5.10.189 and 5.10.190 on the "outer" virtual host, and both Debian 12 running libvirt, and Proxmox 7 as "inner" hosts. For Debian, the nested VM fails to start at all, and consumes the entirety of one CPU core. For Proxmox, 100-200MB/second of memory is allocated and never released, and also the guest fails to start. The problems go away when taking the outer host back to 5.10.188.

The processor in question is a Ryzen 7 2700. The kernel revision for Proxmox is 5.15.108-1-pve, and the kernel revision for Debian 12 is 6.1.0-11-amd64. I've run into another person who can confirm that this bug also occurs in the 6.4 series, "somewhere between 6.4.3 and 6.4.9", 6.4.9 being the likely culprit.

Please let me know if you need any other information. 

I apologize for bothering an entire mailing list, Greg's email bot told me to.

Re: 5.10.189 and 5.10.190 breaks nested virtualization

[Sean Christopherson]

On Wed, Aug 16, 2023, Dr. David Alan Gilbert wrote:
> * Blair Strater (strater@protonmail.com) wrote:
> > Per the request at https://lwn.net/Articles/940798/, I'm reaching out to
> > let you know that this patch breaks nested virtualization on AMD
> > processors.
> > 
> > I've tested 5.10.189 and 5.10.190 on the "outer" virtual host, and both
> > Debian 12 running libvirt, and Proxmox 7 as "inner" hosts. For Debian, the
> > nested VM fails to start at all, and consumes the entirety of one CPU core.
> > For Proxmox, 100-200MB/second of memory is allocated and never released,
> > and also the guest fails to start. The problems go away when taking the
> > outer host back to 5.10.188.
> > 
> > The processor in question is a Ryzen 7 2700. The kernel revision for
> > Proxmox is 5.15.108-1-pve, and the kernel revision for Debian 12 is
> > 6.1.0-11-amd64. I've run into another person who can confirm that this bug
> > also occurs in the 6.4 series, "somewhere between 6.4.3 and 6.4.9", 6.4.9
> > being the likely culprit.
> > 
> > Please let me know if you need any other information. 
> > 
> > I apologize for bothering an entire mailing list, Greg's email bot told me to.
> 
> cc'd in Vitaly (who I notice was working another regression bug in
> the recent stables), and Sean (who I notice has an L2 patch in the
> 5.10.189..190 set).

Does running with "spec_rstack_overflow=off" fix things for you?  If so, can you
then try testing the fix for the guest RFLAGS corruption[1]?  It's a bit of a long
shot, but I'm hoping we'll get lucky and all of these nested SVM errors[2] are
just weird symptoms of branches going awry.

[1] https://lore.kernel.org/all/20230811155255.250835-1-seanjc@google.com
[2] https://bugzilla.kernel.org/show_bug.cgi?id=217796